The anomalous Arequipa Massif, along the coast of southwestern Peru, is part of an exotic terrane... more The anomalous Arequipa Massif, along the coast of southwestern Peru, is part of an exotic terrane (The Arequipa- Antofalla Basement) that was likely emplaced along the margin of Amazonia at ca. 1 Ga. As such, it may be a tectonic tracer left behind by the craton that collided with Amazonia during the ca. 1 Ga Sunsas Orogeny. Correlation of the massif with its "parent craton" could constrain the position of Amazonia within the Mesoproterozoic supercontinent Rodinia. Better characterization of the "pre-1 Ga" tectonic evolution of the Arequipa Massif is necessary to evaluate potential parent cratons. Previous geochronologic work has constrained the timing of Paleoproterozoic events. Bimodal magmatism occurred between 2.02-1.82 Ga. This granite and gabbro was metamorphosed between 1.82 and 1.79 Ga. The resulting gneisses were intruded by granite at ca. 1.79 Ga. The entire sequence was metamorphosed ca. 1 Ga, generating a planar fabric in the younger granite. Based on...
ABSTRACT Precambrian age-provinces have coherent Pb isotopic signatures that commonly differ from... more ABSTRACT Precambrian age-provinces have coherent Pb isotopic signatures that commonly differ from one craton to another. These distinctions have been exploited in paleogeographic reconstruction to test the affinities of potentially exotic terranes with host cratons or potential parent cratons. However, the global variability and distribution of these signatures was not well defined. Based on a compilation of whole-rock Pb isotopic compositions of Precambrian juvenile intrusions we conclude that; 1) Rocks in ca. 2.8-2.6 Ga, ca. 2.0-1.8 Ga, and ca. 1.3-1.0 Ga age-provinces in eastern Laurentia have coherent Pb isotopic signatures that vary systematically as if they were derived from the same homogeneous mantle reservoir that evolved with a constant mu -value. 2) Signatures of ca. 1.3-1.0 Ga age provinces from Amazonia, Antarctica, Australia, Baltica, Kalahari, Oaxaca, and Laurentia overlap each other and define two global Pb isotopic signatures suggesting the presence of at least two large, isotopically-distinct mantle reservoirs during the Mesoproterozoic. Pb isotopic compositions of modern oceanic crust indicate the existence of two Mesozoic/Cenozoic, isotopically-distinct mantle reservoirs, referred to as "normal" mantle in the northern hemisphere and DUPAL mantle in the southern hemisphere. Compositions of Precambrian juvenile intrusions indicate that similar, long-lived reservoirs may have existed earlier in Earth's history.
The Ash Mountain Complex (AMC) in the western Sierra Nevada batholith (SNB; California, USA) is a... more The Ash Mountain Complex (AMC) in the western Sierra Nevada batholith (SNB; California, USA) is an exposure of six compositionally diverse intrusive lithologies with clear crosscutting relationships that permit a focused investigation of magma source characteristics and the relative roles of petro genetic processes on the evolution of the system. We use new fi eld observations, zircon U-Pb dates, major and trace element data, and Sr-Nd-Pb isotopic data to develop a model that can be applied to similar SNB intrusive suites. Stage 1 units, emplaced ca. 105 Ma, consist of two gabbros, a gabbrodiorite, and a granite. Stage 2 and stage 3 units were emplaced ca. 104 Ma and ca. 103 Ma, respectively, and are granites. We suggest that stage 1 gabbroids were derived by partial melting of lithospheric mantle, whereas coeval felsic magmas were derived by partial melting of a mafi c, juvenile crustal source. Stage 2 and stage 3 granitoids were derived from similar sources that generated stage 1 granitoids, but there was greater input from evolved crust. Fractionation and/or assimilation played only a minor role in system evolution. Past studies of SNB magmas have come to confl icting conclusions about the petrogenesis of intermediate magmas that dominate the batholith; we hypothesize that mafi c and felsic end members of the AMC could represent end members in mixing processes that generate these magmas. The timing of emplacement of the AMC coincides with a transition of magmatic style in the SNB, from smaller volume magmatic suites with mixed mantle and crustal sources to larger volume magmatic suites derived from greater proportions of crust.
Eclogite is typically exhumed by either entrainment of large blocks in an accretionary mélange or... more Eclogite is typically exhumed by either entrainment of large blocks in an accretionary mélange or by extensional unroofing of a regional eclogite-facies terrane. The Ashe Metamorphic Suite (AMS) in the Blue Ridge Mountains of North Carolina contains kilometer-scale eclogite blocks within what appears to be an accretionary wedge that has been metamorphosed to amphibolite facies. The eclogite-facies mineral lineation in
CRATER AND THE ABSENCE OF AN IMPACTOR SIGNATURE AT WEST CLEARWATER. R.T. Daly, P.H. Schultz, S.L.... more CRATER AND THE ABSENCE OF AN IMPACTOR SIGNATURE AT WEST CLEARWATER. R.T. Daly, P.H. Schultz, S.L. Loewy, J.C. Lassiter, L.M. Thompson, and J.G. Spray, Department of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook St., Providence, RI 02912, Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Planetary and Space Science Centre, University of New Brunswick, NB E3B 5A3, Canada. Email: [email protected].
Precambrian age-provinces commonly have internally coherent Pb isotopic signatures. Compilation o... more Precambrian age-provinces commonly have internally coherent Pb isotopic signatures. Compilation of whole-rock Pb isotopic compositions of Precambrian juvenile igneous rocks reveals that: 1) ca. 2.8-2.6 Ga, ca. 2.0-1.8 Ga and ca. 1.3-1.0 Ga age-provinces in eastern Laurentia (North America, Greenland, and northwestern Scotland) display a systematic variation in Pb isotopic signatures that indicate they were derived from a single, homogeneous mantle reservoir that evolved with a constant μ-value from 2.8 to 1.0 Ga, and 2) signatures of ca. 1.3-1.0 Ga age-provinces define two distinct, global Pb isotopic signatures, indicating that at least two, large, isotopically distinct mantle reservoirs existed during the Mesoproterozoic. Specifically, ca. 1.3-1.0 Ga
Abstract The Middle Stone Age (MSA) is important to studies of human evolution because it witness... more Abstract The Middle Stone Age (MSA) is important to studies of human evolution because it witnessed the origin of modern Homo sapiens and its dispersal out of Africa and across the rest of the Old World. Obtaining accurate ages for these and other events that occurred during this time interval is of critical importance. Ostrich eggshell (OES) is commonly found in MSA sites, and OES calcite can be dated by radiocarbon and U-series geochronologic techniques. It is important to understand how both techniques are potentially compromised when the eggshell’s pore clusters and intercrystalline microporosity are infilled with younger calcite and/or soil-derived detrital materials. We investigated this question by using high-resolution X-ray computed tomography (HRXCT) to reveal the extent and three-dimensional structure of the pore clusters, and element mapping to evaluate concentrations of detrital elements in modern versus ancient OES within the pore clusters and the crystal (outer) and cone (inner) layers relative to the palisade (central) layer. HRXCT results support earlier findings that OES pores are generally composed of one to several openings on the cone surface that develop into a complex three-dimensional cluster of anastomosing and branching pores that extend through the palisade layer to exit on the crystal surface. These hollow pore clusters provide a considerable total volume for the potential ingress of detrital materials. Element mapping revealed high concentrations of detrital mineral indicator elements (Al, Mg) in ancient relative to modern OES, and in the cone and crystal layers relative to the palisade layer. Within the palisade layer of ancient OES, there are relatively high detrital mineral indicator element concentrations within filled pore clusters. Extracted pore cluster infill from the palisade layer yields significantly higher and more variable concentrations of 232Th (2,200-10,000 ppt) and 238U (35–54 ppb) relative to the non-porous palisade layer (150–3,100 ppt 232Th, and 19–25 ppb 238U). Based on these results, we developed a mechanical preparation method that combines the removal of the pore cluster infill with the more common removal of the cone and crystal layers. Using this method, U–Th analyses of four OES from an MSA archaeological site in NW Ethiopia yielded a mean age of 75.7±4.7 ka (2 SD). The inclusion of the pore cluster infill with the palisade in a different OES from the same site produced a much younger age of 53±1.2 ka (1 SE). Splits of several of these same OES samples prepared by this new method yield AMS 14C ages that are beyond the range of 14C dating (∼50 ka), while ∼60% of the OES analyses of the palisade layer that retained the pore cluster infill yielded much younger ages (27–38 ka). Our results demonstrate that dating OES that retains the pore cluster infill shifts both U–Th and 14C analyses to younger ages. In order to obtain more precise and accurate ages, we recommend that preparation of OES for both U-series and 14C dating include the mechanical removal of both the pore cluster infill and the cone and crystal layers in order to more fully account for the incorporation of Th, U, and C from detrital and authigenic minerals within the OES structure.
The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence b... more The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high-pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado-Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multimineral isochron 87 Rb/ 86 Sr dating on carefully selected mineral samples from high-pressure metamorphic rocks in the NFC to better constrain the timing of high-pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multimineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1 6 1.1, 16.0 6 0.3, and 13.3 6 1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87 Rb/ 86 Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu-Hf ages on garnet and U-Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40 Ar/ 39 Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid-Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing.
Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major ... more Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major and trace element compositions, relict mineral (spinel and pyroxene) compositions, and stable isotope (O, H, Cl) compositions with the goal of determining protolith origin and subsequent serpentinizing fluid sources in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n = 12). Three samples from Cuesta Ridge (part of the Coast Range ophiolite) were also analyzed for comparison. Serpentinites from Cuesta Ridge have flat to U-shaped chondrite-normalized REE patterns and spinels with Cr# values > 0.60 implying a supra-subduction zone origin. In contrast, Franciscan serpentinites west of Cuesta Ridge and Tiburon Peninsula have positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Most relict spinels have low Cr# values (b0.3) and relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. Franciscan serpentinite samples from south of San Francisco and near Healdsburg have U-shaped REE patterns and spinel compositions that lie within the forearc peridotite field with some overlap into the abyssal field and are of more ambiguous origin. All samples are high in fluid-mobile elements with remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to involvement of highly oxidizing fluids resulting in the preferential scavenging of Ce and Y by ferromanganese oxyhydroxides during serpentinization. All samples (except those south of San Francisco) have δ 18 O values of +5.4 to +7.9‰, typical values for oceanic serpentinites formed via low-T seawater hydration on the seafloor. δD values of all samples are extremely low (−107 to −90‰), likely the result of post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high δ 18 O values (+7.2 to +9.5‰) likely due to low-T interaction with meteoric water at high fluid-rock ratios. Most of the serpentinites have δ 37 Cl values between +0.2 and +0.9‰, typical values for serpentinites formed by interaction with seawater. Exceptions are those from the San Andreas fault and one sample from Cuesta Ridge with a high δ 37 Cl value (+1.7‰) possibly from interaction with a slab-derived fluid.
O skarn garnets 18 δ magmatic arc from low-A detailed record of shallow hydrothermal fluid flow i... more O skarn garnets 18 δ magmatic arc from low-A detailed record of shallow hydrothermal fluid flow in the Sierra Nevada Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science.
Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1... more Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1112 ± 4 Ma, are identical in age to both the Umkondo large igneous province (LIP) of the Kalahari craton (southern Africa) and the early Keweenawan LIP of Laurentia (North America). Although marine and satellite data demonstrate that Coats Land was close to Kalahari within the Gondwana supercontinent, the Coats Land rocks yield Pb isotope compositions strikingly distinct from those of the Umkondo province, yet indistinguishable from rocks of the Keweenawan province. The anorogenic Red Bluff granitic suite, along the present-day southern Laurentian margin in the Franklin Mountains (Texas, USA), is of comparable age, general rock type, and Pb isotope composition to rocks of Coats Land and may provide a piercing point for a Coats Land-Laurentia link. Paleomagnetic poles permit the Coats Land block to be close to this part of Laurentia ca. 1100 Ma and allow juxtaposition of Kalahari and southern Laurentia ca. 1000 Ma. The Coats Land crustal block may therefore be a critical tectonic tracer for placing Laurentia within late Mesoproterozoic and Neoproterozoic paleogeographic reconstructions. If this hypothesis is correct, Laurentia collided with the Kalahari craton along Antarctica's Maud orogen, which would represent a continuation of the ca. 1000 Ma Grenville orogen of eastern and southern Laurentia.
The conventional view that the basement of the southern and central Appalachians represents juven... more The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the fi nal stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm-Nd isotopic data are presented from Meso protero zoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee-western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (T DM ) ages >2.0 Ga, and the most elevated 207 Pb/ 204 Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better defi ne the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province.
In the southern Sierra Nevada, intrusion of the Cretaceous Empire quartz diorite into the calcare... more In the southern Sierra Nevada, intrusion of the Cretaceous Empire quartz diorite into the calcareous rocks of the Mineral King pendant resulted in the spectacular development of garnetite and related skarn. Massive garnets (up to 10-15 cm in diameter) and other field relationships at this locality require unusual metamorphic conditions, such as the temperature of intrusion, the composition of the wall rocks, fluid and volatile compositions, and pressure. With a limited range in silica content (60-64 wt %, up to 71 wt %), the Empire quartz diorite is one of the plutons that make up the Kings sequence, which surrounded and intruded the pendant. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analysis of the Empire pluton yielded an age of ca. 109±0.8 Ma, revising the previous age of 107 Ma (Busby-Spera and Saleeby, 1987). This age is synchronous with other quartz diorite magmatism in the Sierra, but well inboard of the Foothills quartz-diorite belt of Moore (1959). In addition, the high Ti concentration (28±6 ppm) of zircons from the magnetite bearing quartz diorite indicate magmatic temperatures of 890±23°C [Ferry and Watson (2007) calibration; alphaTi = 0.5, alphaSi = 0.7], the hottest Ti-in-zircon temperature yet reported from a Sierran granitoid. At Empire Mountain erosion has exposed an excellent three-dimensional view of the relationships between the Empire quartz diorite, skarns, and the protolith calc-silicate rocks. There is a transitional zone between the intact roof pendant and the quartz diorite across tens of meters in the cupola zone of the pluton. Throughout this zone, quartz diorite pervasively intrudes what appear to be products of spalling from the overlying pendant. These calc-silicate xenoliths become increasingly garnet-rich toward their margins, suggesting significant metasomatism. At the top of this zone, apparent forceful injection of quartz diorite caused the propagation of fractures into the pendant. The exposed pendant consists almost entirely of garnet and associated skarn minerals, including epidote and quartz. The intensity of skarn formation at the contact suggests extraordinary conditions of metamorphism, which most likely resulted from the extreme temperature of intrusion and the metasomatic interactions between the quartz diorite and the roof pendant.
Geological Society, London, Special Publications, 2004
The Catskill Delta Complex of western New York State contains fractured Upper Devonian black shal... more The Catskill Delta Complex of western New York State contains fractured Upper Devonian black shales throughout a 300 km-transect from the more distal, somewhat shallower, deposits of the western region of the state eastward to more proximal and more deeply buried deposits. Each black shale unit grades upward into organically lean grey shale and abruptly overlies another grey shale unit. Within each black shale-grey shale sequence, ENE-trending vertical joints, interpreted to be hydraulic fractures, are best developed (i.e. more closely and uniformly spaced) in the organic-rich shale. Moreover, the density of ENE joints diminishes up-section through each black shale unit, as does the total organic carbon (TOC) content. While ENE joints are less well developed outside the black shale intervals, joints that formed during the Alleghanian orogeny (NW-trending) are found throughout the Upper Devonian shale sequence. Both sets are best developed in black shales in the distal delta sequence...
Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laure... more Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post-1000 Ma. U-Pb zircon ages for felsic and associated mafi c intrusions confi rm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of defor ma tion and metamorphic pulses related to Knoydartian orogenesis of the host Moine rocks. Additional U-Pb zircon and monazite data, and 40 Ar/ 39 Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458-446 Ma and ca. 426 Ma during the Caledonian orogenic cycle. The presence of early Neoproterozoic silici clastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America refl ect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with the inferred development of convergent platemargin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession.
The anomalous Arequipa Massif, along the coast of southwestern Peru, is part of an exotic terrane... more The anomalous Arequipa Massif, along the coast of southwestern Peru, is part of an exotic terrane (The Arequipa- Antofalla Basement) that was likely emplaced along the margin of Amazonia at ca. 1 Ga. As such, it may be a tectonic tracer left behind by the craton that collided with Amazonia during the ca. 1 Ga Sunsas Orogeny. Correlation of the massif with its "parent craton" could constrain the position of Amazonia within the Mesoproterozoic supercontinent Rodinia. Better characterization of the "pre-1 Ga" tectonic evolution of the Arequipa Massif is necessary to evaluate potential parent cratons. Previous geochronologic work has constrained the timing of Paleoproterozoic events. Bimodal magmatism occurred between 2.02-1.82 Ga. This granite and gabbro was metamorphosed between 1.82 and 1.79 Ga. The resulting gneisses were intruded by granite at ca. 1.79 Ga. The entire sequence was metamorphosed ca. 1 Ga, generating a planar fabric in the younger granite. Based on...
ABSTRACT Precambrian age-provinces have coherent Pb isotopic signatures that commonly differ from... more ABSTRACT Precambrian age-provinces have coherent Pb isotopic signatures that commonly differ from one craton to another. These distinctions have been exploited in paleogeographic reconstruction to test the affinities of potentially exotic terranes with host cratons or potential parent cratons. However, the global variability and distribution of these signatures was not well defined. Based on a compilation of whole-rock Pb isotopic compositions of Precambrian juvenile intrusions we conclude that; 1) Rocks in ca. 2.8-2.6 Ga, ca. 2.0-1.8 Ga, and ca. 1.3-1.0 Ga age-provinces in eastern Laurentia have coherent Pb isotopic signatures that vary systematically as if they were derived from the same homogeneous mantle reservoir that evolved with a constant mu -value. 2) Signatures of ca. 1.3-1.0 Ga age provinces from Amazonia, Antarctica, Australia, Baltica, Kalahari, Oaxaca, and Laurentia overlap each other and define two global Pb isotopic signatures suggesting the presence of at least two large, isotopically-distinct mantle reservoirs during the Mesoproterozoic. Pb isotopic compositions of modern oceanic crust indicate the existence of two Mesozoic/Cenozoic, isotopically-distinct mantle reservoirs, referred to as "normal" mantle in the northern hemisphere and DUPAL mantle in the southern hemisphere. Compositions of Precambrian juvenile intrusions indicate that similar, long-lived reservoirs may have existed earlier in Earth's history.
The Ash Mountain Complex (AMC) in the western Sierra Nevada batholith (SNB; California, USA) is a... more The Ash Mountain Complex (AMC) in the western Sierra Nevada batholith (SNB; California, USA) is an exposure of six compositionally diverse intrusive lithologies with clear crosscutting relationships that permit a focused investigation of magma source characteristics and the relative roles of petro genetic processes on the evolution of the system. We use new fi eld observations, zircon U-Pb dates, major and trace element data, and Sr-Nd-Pb isotopic data to develop a model that can be applied to similar SNB intrusive suites. Stage 1 units, emplaced ca. 105 Ma, consist of two gabbros, a gabbrodiorite, and a granite. Stage 2 and stage 3 units were emplaced ca. 104 Ma and ca. 103 Ma, respectively, and are granites. We suggest that stage 1 gabbroids were derived by partial melting of lithospheric mantle, whereas coeval felsic magmas were derived by partial melting of a mafi c, juvenile crustal source. Stage 2 and stage 3 granitoids were derived from similar sources that generated stage 1 granitoids, but there was greater input from evolved crust. Fractionation and/or assimilation played only a minor role in system evolution. Past studies of SNB magmas have come to confl icting conclusions about the petrogenesis of intermediate magmas that dominate the batholith; we hypothesize that mafi c and felsic end members of the AMC could represent end members in mixing processes that generate these magmas. The timing of emplacement of the AMC coincides with a transition of magmatic style in the SNB, from smaller volume magmatic suites with mixed mantle and crustal sources to larger volume magmatic suites derived from greater proportions of crust.
Eclogite is typically exhumed by either entrainment of large blocks in an accretionary mélange or... more Eclogite is typically exhumed by either entrainment of large blocks in an accretionary mélange or by extensional unroofing of a regional eclogite-facies terrane. The Ashe Metamorphic Suite (AMS) in the Blue Ridge Mountains of North Carolina contains kilometer-scale eclogite blocks within what appears to be an accretionary wedge that has been metamorphosed to amphibolite facies. The eclogite-facies mineral lineation in
CRATER AND THE ABSENCE OF AN IMPACTOR SIGNATURE AT WEST CLEARWATER. R.T. Daly, P.H. Schultz, S.L.... more CRATER AND THE ABSENCE OF AN IMPACTOR SIGNATURE AT WEST CLEARWATER. R.T. Daly, P.H. Schultz, S.L. Loewy, J.C. Lassiter, L.M. Thompson, and J.G. Spray, Department of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook St., Providence, RI 02912, Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Planetary and Space Science Centre, University of New Brunswick, NB E3B 5A3, Canada. Email: [email protected].
Precambrian age-provinces commonly have internally coherent Pb isotopic signatures. Compilation o... more Precambrian age-provinces commonly have internally coherent Pb isotopic signatures. Compilation of whole-rock Pb isotopic compositions of Precambrian juvenile igneous rocks reveals that: 1) ca. 2.8-2.6 Ga, ca. 2.0-1.8 Ga and ca. 1.3-1.0 Ga age-provinces in eastern Laurentia (North America, Greenland, and northwestern Scotland) display a systematic variation in Pb isotopic signatures that indicate they were derived from a single, homogeneous mantle reservoir that evolved with a constant μ-value from 2.8 to 1.0 Ga, and 2) signatures of ca. 1.3-1.0 Ga age-provinces define two distinct, global Pb isotopic signatures, indicating that at least two, large, isotopically distinct mantle reservoirs existed during the Mesoproterozoic. Specifically, ca. 1.3-1.0 Ga
Abstract The Middle Stone Age (MSA) is important to studies of human evolution because it witness... more Abstract The Middle Stone Age (MSA) is important to studies of human evolution because it witnessed the origin of modern Homo sapiens and its dispersal out of Africa and across the rest of the Old World. Obtaining accurate ages for these and other events that occurred during this time interval is of critical importance. Ostrich eggshell (OES) is commonly found in MSA sites, and OES calcite can be dated by radiocarbon and U-series geochronologic techniques. It is important to understand how both techniques are potentially compromised when the eggshell’s pore clusters and intercrystalline microporosity are infilled with younger calcite and/or soil-derived detrital materials. We investigated this question by using high-resolution X-ray computed tomography (HRXCT) to reveal the extent and three-dimensional structure of the pore clusters, and element mapping to evaluate concentrations of detrital elements in modern versus ancient OES within the pore clusters and the crystal (outer) and cone (inner) layers relative to the palisade (central) layer. HRXCT results support earlier findings that OES pores are generally composed of one to several openings on the cone surface that develop into a complex three-dimensional cluster of anastomosing and branching pores that extend through the palisade layer to exit on the crystal surface. These hollow pore clusters provide a considerable total volume for the potential ingress of detrital materials. Element mapping revealed high concentrations of detrital mineral indicator elements (Al, Mg) in ancient relative to modern OES, and in the cone and crystal layers relative to the palisade layer. Within the palisade layer of ancient OES, there are relatively high detrital mineral indicator element concentrations within filled pore clusters. Extracted pore cluster infill from the palisade layer yields significantly higher and more variable concentrations of 232Th (2,200-10,000 ppt) and 238U (35–54 ppb) relative to the non-porous palisade layer (150–3,100 ppt 232Th, and 19–25 ppb 238U). Based on these results, we developed a mechanical preparation method that combines the removal of the pore cluster infill with the more common removal of the cone and crystal layers. Using this method, U–Th analyses of four OES from an MSA archaeological site in NW Ethiopia yielded a mean age of 75.7±4.7 ka (2 SD). The inclusion of the pore cluster infill with the palisade in a different OES from the same site produced a much younger age of 53±1.2 ka (1 SE). Splits of several of these same OES samples prepared by this new method yield AMS 14C ages that are beyond the range of 14C dating (∼50 ka), while ∼60% of the OES analyses of the palisade layer that retained the pore cluster infill yielded much younger ages (27–38 ka). Our results demonstrate that dating OES that retains the pore cluster infill shifts both U–Th and 14C analyses to younger ages. In order to obtain more precise and accurate ages, we recommend that preparation of OES for both U-series and 14C dating include the mechanical removal of both the pore cluster infill and the cone and crystal layers in order to more fully account for the incorporation of Th, U, and C from detrital and authigenic minerals within the OES structure.
The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence b... more The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high-pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado-Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multimineral isochron 87 Rb/ 86 Sr dating on carefully selected mineral samples from high-pressure metamorphic rocks in the NFC to better constrain the timing of high-pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multimineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1 6 1.1, 16.0 6 0.3, and 13.3 6 1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87 Rb/ 86 Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu-Hf ages on garnet and U-Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40 Ar/ 39 Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid-Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing.
Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major ... more Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major and trace element compositions, relict mineral (spinel and pyroxene) compositions, and stable isotope (O, H, Cl) compositions with the goal of determining protolith origin and subsequent serpentinizing fluid sources in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n = 12). Three samples from Cuesta Ridge (part of the Coast Range ophiolite) were also analyzed for comparison. Serpentinites from Cuesta Ridge have flat to U-shaped chondrite-normalized REE patterns and spinels with Cr# values > 0.60 implying a supra-subduction zone origin. In contrast, Franciscan serpentinites west of Cuesta Ridge and Tiburon Peninsula have positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Most relict spinels have low Cr# values (b0.3) and relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. Franciscan serpentinite samples from south of San Francisco and near Healdsburg have U-shaped REE patterns and spinel compositions that lie within the forearc peridotite field with some overlap into the abyssal field and are of more ambiguous origin. All samples are high in fluid-mobile elements with remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to involvement of highly oxidizing fluids resulting in the preferential scavenging of Ce and Y by ferromanganese oxyhydroxides during serpentinization. All samples (except those south of San Francisco) have δ 18 O values of +5.4 to +7.9‰, typical values for oceanic serpentinites formed via low-T seawater hydration on the seafloor. δD values of all samples are extremely low (−107 to −90‰), likely the result of post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high δ 18 O values (+7.2 to +9.5‰) likely due to low-T interaction with meteoric water at high fluid-rock ratios. Most of the serpentinites have δ 37 Cl values between +0.2 and +0.9‰, typical values for serpentinites formed by interaction with seawater. Exceptions are those from the San Andreas fault and one sample from Cuesta Ridge with a high δ 37 Cl value (+1.7‰) possibly from interaction with a slab-derived fluid.
O skarn garnets 18 δ magmatic arc from low-A detailed record of shallow hydrothermal fluid flow i... more O skarn garnets 18 δ magmatic arc from low-A detailed record of shallow hydrothermal fluid flow in the Sierra Nevada Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science.
Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1... more Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1112 ± 4 Ma, are identical in age to both the Umkondo large igneous province (LIP) of the Kalahari craton (southern Africa) and the early Keweenawan LIP of Laurentia (North America). Although marine and satellite data demonstrate that Coats Land was close to Kalahari within the Gondwana supercontinent, the Coats Land rocks yield Pb isotope compositions strikingly distinct from those of the Umkondo province, yet indistinguishable from rocks of the Keweenawan province. The anorogenic Red Bluff granitic suite, along the present-day southern Laurentian margin in the Franklin Mountains (Texas, USA), is of comparable age, general rock type, and Pb isotope composition to rocks of Coats Land and may provide a piercing point for a Coats Land-Laurentia link. Paleomagnetic poles permit the Coats Land block to be close to this part of Laurentia ca. 1100 Ma and allow juxtaposition of Kalahari and southern Laurentia ca. 1000 Ma. The Coats Land crustal block may therefore be a critical tectonic tracer for placing Laurentia within late Mesoproterozoic and Neoproterozoic paleogeographic reconstructions. If this hypothesis is correct, Laurentia collided with the Kalahari craton along Antarctica's Maud orogen, which would represent a continuation of the ca. 1000 Ma Grenville orogen of eastern and southern Laurentia.
The conventional view that the basement of the southern and central Appalachians represents juven... more The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the fi nal stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm-Nd isotopic data are presented from Meso protero zoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee-western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (T DM ) ages >2.0 Ga, and the most elevated 207 Pb/ 204 Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better defi ne the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province.
In the southern Sierra Nevada, intrusion of the Cretaceous Empire quartz diorite into the calcare... more In the southern Sierra Nevada, intrusion of the Cretaceous Empire quartz diorite into the calcareous rocks of the Mineral King pendant resulted in the spectacular development of garnetite and related skarn. Massive garnets (up to 10-15 cm in diameter) and other field relationships at this locality require unusual metamorphic conditions, such as the temperature of intrusion, the composition of the wall rocks, fluid and volatile compositions, and pressure. With a limited range in silica content (60-64 wt %, up to 71 wt %), the Empire quartz diorite is one of the plutons that make up the Kings sequence, which surrounded and intruded the pendant. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analysis of the Empire pluton yielded an age of ca. 109±0.8 Ma, revising the previous age of 107 Ma (Busby-Spera and Saleeby, 1987). This age is synchronous with other quartz diorite magmatism in the Sierra, but well inboard of the Foothills quartz-diorite belt of Moore (1959). In addition, the high Ti concentration (28±6 ppm) of zircons from the magnetite bearing quartz diorite indicate magmatic temperatures of 890±23°C [Ferry and Watson (2007) calibration; alphaTi = 0.5, alphaSi = 0.7], the hottest Ti-in-zircon temperature yet reported from a Sierran granitoid. At Empire Mountain erosion has exposed an excellent three-dimensional view of the relationships between the Empire quartz diorite, skarns, and the protolith calc-silicate rocks. There is a transitional zone between the intact roof pendant and the quartz diorite across tens of meters in the cupola zone of the pluton. Throughout this zone, quartz diorite pervasively intrudes what appear to be products of spalling from the overlying pendant. These calc-silicate xenoliths become increasingly garnet-rich toward their margins, suggesting significant metasomatism. At the top of this zone, apparent forceful injection of quartz diorite caused the propagation of fractures into the pendant. The exposed pendant consists almost entirely of garnet and associated skarn minerals, including epidote and quartz. The intensity of skarn formation at the contact suggests extraordinary conditions of metamorphism, which most likely resulted from the extreme temperature of intrusion and the metasomatic interactions between the quartz diorite and the roof pendant.
Geological Society, London, Special Publications, 2004
The Catskill Delta Complex of western New York State contains fractured Upper Devonian black shal... more The Catskill Delta Complex of western New York State contains fractured Upper Devonian black shales throughout a 300 km-transect from the more distal, somewhat shallower, deposits of the western region of the state eastward to more proximal and more deeply buried deposits. Each black shale unit grades upward into organically lean grey shale and abruptly overlies another grey shale unit. Within each black shale-grey shale sequence, ENE-trending vertical joints, interpreted to be hydraulic fractures, are best developed (i.e. more closely and uniformly spaced) in the organic-rich shale. Moreover, the density of ENE joints diminishes up-section through each black shale unit, as does the total organic carbon (TOC) content. While ENE joints are less well developed outside the black shale intervals, joints that formed during the Alleghanian orogeny (NW-trending) are found throughout the Upper Devonian shale sequence. Both sets are best developed in black shales in the distal delta sequence...
Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laure... more Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post-1000 Ma. U-Pb zircon ages for felsic and associated mafi c intrusions confi rm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of defor ma tion and metamorphic pulses related to Knoydartian orogenesis of the host Moine rocks. Additional U-Pb zircon and monazite data, and 40 Ar/ 39 Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458-446 Ma and ca. 426 Ma during the Caledonian orogenic cycle. The presence of early Neoproterozoic silici clastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America refl ect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with the inferred development of convergent platemargin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession.
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