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31.
DOWNES HILARY; MACDONALD RAY; UPTON BRIAN G. J.; COX KEITH G.; BODINIER JEAN-LOUIS; MASON PAUL R. D.; JAMES DODIE; HILL PETER G.; HEARN B. CARTER JR 《Journal of Petrology》2004,45(8):1631-1662
Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al2O3contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al2O3 and TiO2) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have 143Nd/144Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low 143Nd/144Nd values (0·5113)and extremely high 87Sr/86Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their SrNdisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette 相似文献
32.
RAY Durbar BANERJEE Ranadip MAZUMDER Anin PAROPKARI Anil L. MUKHOPADHYAY Subir BALAKRISHNAN Srinivasan 《海洋学报(英文版)》2018,37(4):22-33
Polymetallic sulfides from two hydrothermal chimneys and talus deposit from the Vienna Woods field of Manus Basin were studied for mineralogy, elemental composition, and S-isotope ratio to understand their evolution. The factors including the nature of source fluid, mineral paragenesis, and related geochemical processes have been discussed. Mineralogy and elemental concentration of Cu and Fe-rich large chimney at the central part of this hydrothermal field was completely different from the smaller Zn-rich peripheral chimney and Fe-rich talus deposit, suggesting the variable degree of alterations generate physico-chemically different source fluids responsible for these hydrothermal structures. Similarly, S-isotope ratios also indicate chemically diverse fluids and different modes of precipitation were involved in their evolution. Distinct mineral zonings and associated elemental and isotopic compositions within individual deposit confirm paragenetic shifts were involved during their growth process. 相似文献
33.
SOHINI?GANGULYEmail author JYOTISANKAR?RAY CHRISTIAN?KOEBERL THEODOROS?NTAFLOS MOUSUMI?BANERJEE 《Journal of Earth System Science》2012,121(1):91-108
Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province
(EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints
and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG)
sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector.
A three-tier classification scheme has been adopted for the characterization and classification of individual lava flows.
Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ).
The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively.
The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing.
The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed
petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development
of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny
plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range
∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes.
Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite
component. Pyroxene phenocrysts are characterized by a prominent Ti-enrichment. Phenocrystal plagioclase grains are calcic
(An52.7–An72.9), whereas groundmass plagioclase are relatively sodic (An39.2–An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals
a temperature span of 850°C to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range
from 1040°–1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption. 相似文献
34.
Serpentinites from the inside corner high(6°38.5′S/68°19.34′E)from the Northern Central Indian Ridge(NCIR)are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies.‘Mesh rim',‘window',‘hourglass' and‘bastite' are the most common textures displayed by both chrysotile and lizardite.Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation.The relatively high abundance of chrysotile and lizardite suggest their close association and formation at a temperature below 250℃. Abundant‘mesh rim'and‘bastite'texture and variegated matrix reveal that the present serpentinites might have formed due to the interaction of harzburgite and seawater.Positive Eu anomaly(Eu/Eu~* up to 3.38),higher La/Sm(up to 4.40)and Nb/La(up to 6.34)ratios suggest substantial hydrothermal influence during the formation of the serpentinites. 相似文献
35.
Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley 总被引:13,自引:1,他引:13
Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO2 conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalitemagnetitequartz buffer)and at high fF2 inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite [molar (Na2O+ K2O)/Al2O3 (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A 相似文献
36.
Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California 总被引:1,自引:0,他引:1
Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level. 相似文献
37.
JYOTISANKAR RAY ABHISHEK SAHA SOHINI GANGULY V BALARAM A KESHAV KRISHNA SAMPA HAZRA 《Journal of Earth System Science》2011,120(3):459-473
The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous
plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids
cover an area of about 40 km2 and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton.
Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural
study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence
of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite
and lath-shaped plagioclase (An20–An29). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur
as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to
IUGS system of classification. 相似文献