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1.
The distribution of radiogenic heat production as a function of depth in the Sierra Nevada Batholith, California 总被引：4，自引：0，他引：4
Geochemical analyses and geobarometric determinations have been combined to create a depth vs. radiogenic heat production database for the Sierra Nevada batholith, California. This database shows that mean heat production values first increase, then decrease, with increasing depth. Heat production is 2 μW/m3 within the 3-km-thick volcanic pile at the top of the batholith, below which it increases to an average value of 3.5 μW/m3 at 5.5 km depth, then decreases to 0.5–1 μW/m3 at 15 km depth and remains at these values through the entire crust below 15 km. Below the crust, from depths of 40–125 km, the batholith's root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0.14 μW/m3. This is higher than the rates from most published xenolith studies, but reasonable given the presence of crustal components in the arc root assemblages. The pattern of radiogenic heat production interpreted from the depth vs. heat production database is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. The interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the 35 km thick crust. This requires a very low heat flux from the mantle, which is consistent with a model of cessation of Sierran magmatism during Laramide flat-slab subduction, followed by conductive cooling of the upper mantle for 70 m.y. The heat production variation with depth is principally due to large variations in uranium and thorium concentration; potassium is less variable in concentration within the Sierran crust, and produces relatively little of the heat in high heat production rocks. Because silica content is relatively constant through the upper 30 km of the Sierran batholith, while U, Th, and K concentrations are highly variable, radiogenic heat production does not vary directly with silica content. 相似文献
2.
The basement of the Romanian Carpathians is made of Neoproterozoic to early Paleozoic periGondwanan terranes variably involved in the Variscan orogeny,similarly to other basement terrains of Europe.They were hardly dismembered during the Alpine orogeny and traditionally have their own names in the three Carpathian areas.The Danubian domain of the South Carpathians comprises the Dragsan and Lainici-Paius peri-Amazonian terranes.The Dragsan terrane originated within the ocean surrounding Rodinia and docked with Rodinia at ~800 Ma.It does not contain Cadomian magmatism and consequently it is classified as an Avalonian extra-Cadomian terrane.The Lainici-Paius terrane is a Ganderian fragment strongly modified by Cadomian subduction-related magmatism.It is attached to the Moesia platform.The Tisovita terrane is an ophiolite that marks the boundary between Dragsan and Lainici-Paius terranes.The other basement terranes of the Romanian Carpathians originated close to the Ordovician NorthAfrican orogen,as a result of the eastern Rheic Ocean opening and closure.Except for the Sebes-Lotru terrane that includes a lower metamorphic unit of Cadomian age,all the other terranes(Bretila,Tulghes,Negrisoara and Rebra in the East Carpathians,Somes,Biharia and Baia de Aries in the Apuseni mountains,Fagaras,Leaota,Caras and Pades in the South Carpathians) represent late Cambrian—Ordovician rock assemblages.Their provenance,is probably within paleo-northeast Africa,close to the Arabian-Nubian shield.The late Cambrian-Ordovician terranes are defined here as Carpathian-type terranes.According to their lithostratigraphy and origin,some are of continental margin magmatic arc setting,whereas others formed in rift and back-arc environment and closed to passive continental margin settings.In a paleogeographic reconstruction,the continental margin magmatic arc terranes were first that drifted out,followed by the passive continental margin terranes with the back-arc terranes in their front.They accreted to Laurussia during the Variscan orogeny.Some of them(Sebes-Lotru in South Carpathians and Baia de Aries in Apuseni mountains) underwent eclogite-grade metamorphism.The Danubian terranes,the Bretila terrane and the Somes terrane were intruded by Variscan granitoids. 相似文献
3.
Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada,California 总被引：14，自引：0，他引：14
Lingsen?ZengEmail author Jason?B.?Saleeby Mihai?Ducea 《Contributions to Mineralogy and Petrology》2005,150(4):386-402
We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial 87Sr/86Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125–0.7221). However, the leucosomes have a much wider range of initial εNd values, which range from −6.0 to −11.0, as compared to −8.7 to −11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios (0.15<Rb/Sr<1.0), and initial εNd values. Group B leucosomes have relatively low Rb, Pb, Ba, and K2O contents, Rb/Sr ratios (<0.15), and initial εNd values. The low Rb concentrations and Rb/Sr ratios of the group B leucosomes together suggest that partial melting was dominated by water-saturated or H2O-fluxed melting of quartz + feldspar assemblage with minor involvement of muscovite. Breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low Rb/Sr ratios characterizing both group A and group B leucosomes. In contrast, group A leucosomes have greater contributions from K-feldspar, which is suggested by: (1) their relatively high K concentrations, (2) positive or slightly negative Eu anomalies, and (3) correlation of their Pb and Ba concentrations with K2O contents. It is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as Sm, Nd, Nb, and V between melt products and residues during migmatization. The various degrees of parent/daughter fractionations in the Rb–Sr and Sm–Nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also a potentially important process in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities. 相似文献
4.
Josh A. Calkins George Zandt James Girardi Ken Dueker George E. Gehrels Mihai N. Ducea 《Earth and Planetary Science Letters》2010,289(1-2):145-155
The late Triassic to early Tertiary Coast Mountains Batholith (CMB) of British Columbia provides an ideal locale to study the processes whereby accreted terranes and subduction-related melts interact to form stable continental crust of intermediate to felsic composition and complementary ultramafic residuals. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into the deep-crustal lithologies that are key to understanding the processes of continental growth and evolution. We use a combination of seismic observations and petrologic modeling to construct hypothetical crustal sections at representative locations across the CMB, then test the viability of these sections via forward modeling with synthetic seismic data. The compositions that make up our petrologic forward models are based on calculations using the free energy minimization program Perple_X to predict mineral assemblages at depth for the bulk compositions of exposed plutonic rocks collected in the study area. Seismic data were collected along two transects in west-central British Columbia: a southern line that crossed the CMB near the town of Bella Coola (near 52° N), and a northern line centered on the towns of Terrace and Kitimat (near 54° N). Along both transects, seismic receiver functions reveal high Vp/Vs ratios near the Insular/Intermontane terrane boundary and crustal thickness increasing from 26 ± 3 km to 34 ± 3 km (at the 1 sigma certainty level) from west to east across the Coast Shear Zone (CSZ). On the southern line, we observe an anomalous region of complex receiver functions and diminished Moho signals beneath the central portion of the CMB. Our petrologic and seismic profiles show that observed seismic data from much of the CMB can be well-matched in terms of crustal thickness and structure, average Vp/Vs, and amplitude of the Moho converted phase, without including ultramafic residual material in the lower crust. 相似文献
5.
Conditions and timing of high-pressure Variscan metamorphism in the South Carpathians, Romania 总被引：2，自引：0，他引：2
High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic–Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm–Nd mineral–whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.
A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in “peak” pressures and temperatures of 10.8–22.3 kbar and 545–745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H2O in some samples was substantially reduced, ca. 0.1–0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150–1300 °C, 25.8–29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.
The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P–T–t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins. 相似文献
6.
Relu D. Roban Mihai N. Ducea Vlad I. Mihalcea Ioan Munteanu Victor Barbu Mihaela C. Melinte-Dobrinescu Cornel Olariu Mihai Vlăsceanu 《Basin Research》2023,35(1):244-270
We present new sedimentological, petrographical, palaeontological and detrital zircon U–Pb data on late Oligocene–early Miocene sedimentary rocks of the thin-skinned thrust belt of East Carpathians. These data were acquired to reconstruct the sedimentary routing system for two compositionally different turbidite fans made of the regionally extensive Kliwa and Fusaru formations. On the eastern margin of the Moldavides foreland basin, large low-gradient river systems draining the East European Platform provided well-sorted quartz-rich sand forming deltas on wide shallow shelves and thick Kliwa submarine fans. Due to the westward subduction of a thinned continental plate, the western basin margin was characterized by short, steep-gradient routing systems where sediment transport to deep water was mainly through hyperpycnal flows. The Getic and Bucovinian nappes of the East Carpathians and the exhumed Cretaceous–Early Palaeogene orogenic wedge fed Fusaru fans with poorly sorted lithic sand. The Fusaru fans trend northwards in the foredeep basin having an elongate depocentre, interfingering and then overlapping on the distal part of the Kliwa depositional system due to the eastward advance of the Carpathian fold-and-thrust belt. A smaller sediment input is supplied by southern continental areas (i.e. Moesian Platform, North Dobrogea and potentially the Balkans). In general, the sandstone interfingering between distinct basin floor fan systems is less well documented because the facies would be similar and there are not many systems that have a distinct sediment provenance like Kliwa and Fusaru systems. This case study improves the understanding of regional palaeogeography and sedimentary routing systems and provides observations relevant here or elsewhere on the interfingering turbidite fan systems. 相似文献
7.
Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system 总被引：5，自引：0，他引：5
Nadine McQuarrie Delores Robinson Sean Long Tobgay Tobgay Djordje Grujic George Gehrels Mihai Ducea 《Earth and Planetary Science Letters》2008,272(1-2):105-117
High-precision in-situ ion microprobe (SIMS) oxygen isotope analysis of zircons from two diorite intrusions associated with the late Caledonian Lochnagar pluton in Scotland has revealed large differences in the degree of heterogeneity in zircon δ18O between the diorites. Zircon crystals from the Cul nan Gad diorite (CnG) show a unimodal distribution of oxygen isotope values (δ18O = 6.0 ± 0.6‰ (2σ)) and no or only minor grain-scale variation. Those from the Allt Darrarie diorite (AD1) show a large range in δ18O and an apparent bimodal distribution with modes of 6.6 ± 0.4‰ and 7.3 ± 0.4‰. Variations of up to 1.2‰ occur between and within grains; both an increase and decrease in δ18O with zircon growth has been observed. The δ18O composition of growing zircon can only change if open-system processes affect the magma composition, i.e. if material of contrasting δ18O composition is added to the magma. The variability in AD1 is interpreted to represent a cryptic record of magma mixing. A ‘deep crustal hot zone’ is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks. The overall small number of zircons with mantle-like δ18O values (5.3 ± 0.6‰ (2σ)) in the Lochnagar diorites is largely the product of crustal differentiation rather than crustal growth.
The δ18O of quartz from the CnG and AD1 diorites shows only minor variation (CnG: 10.9 ± 0.5‰ (2σ), AD1: 11.7 ± 0.6‰ (2σ)) within single populations, with no evidence of mixing. Quartz–zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids, and in case of the AD1 diorite after the inferred magma mixing from a homogenised, higher δ18O melt.
High-precision SIMS oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early-stage magma mixing and constraining the compositions and origins of the component magmas. A combination of zircon, quartz and whole-rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration. 相似文献
8.
K–Ca ages of Cambrian glauconites from the Llano uplift, central Texas, were determined in order to re-evaluate the ability
of the K–Ca system to constrain the timing of deposition of sedimentary packages. All of the K–Ca ages presented here were
found to be younger than their stratigraphic ages. In addition to being too young, the K–Ca ages are also highly variable,
ranging in age from Silurian to Permian. The oldest subset of glauconite ages are in agreement with previously published Rb–Sr
ages from the same outcrop and provide further evidence for there having been a post-depositional thermal or recrystallization
event that reset both the Rb–Sr and K–Ca systems. The range of younger glauconite K–Ca ages is similar to the distribution
of available apatite fission track ages for the Llano basement. K–Ca ages are interpreted as thermochronologic data reflecting
partial retention of Ca in thermally fluctuating basin conditions. Estimates of the closure temperature of Ca in glauconite
are found to be 60–90°C for cooling rates of ~0.5–1°C/My. The K–Ca system is potentially useful as a low-temperature thermochronometer
with closure temperatures <100°C for glauconite. 相似文献
9.
The North Dobrogea orogen is a collage of dismembered terrane fragments between the Moesian platform and East European craton (Baltica). It records Alpine and Variscan deformation, magmatism and metamorphism. Its basement comprises three metamorphic complexes (Boclugea, Megina and Orliga) that are separated by tectonic boundaries. Detrital zircon U/Pb ages suggest the Boclugea and Orliga complexes represent two peri-Gondwanan terranes of Avalonian and Cadomian affinities, respectively. The new data clarify the original relationships between the North Dobrogea terranes, and Baltica and Moesia platform. 相似文献
10.
A. Cardona V. A. Valencia G. Bayona J. Duque M. Ducea G. Gehrels C. Jaramillo C. Montes G. Ojeda J. Ruiz 《地学学报》2011,23(1):26-34