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1.
Suryendu Dutta Michael Steiner Santanu Banerjee Bernd-Dietrich Erdtmann Silambuchelvan Jeevankumar Ulrich Mann 《Journal of Earth System Science》2006,115(1):99-112
Chuaria circularis (Walcott 1899) from the Suket Shale of the Vindhyan Supergroup (central India) has been reinvestigated for its morphology
and chemical composition using biostatistics, electron microscopy and pyrolysis-gas chromatography. Morphology and microscopic
investigations provide little clues on the specific biological affinity ofChuaria as numerous preservational artifacts seem to be incorporated. On the contrary, the predominance of η aliphatic pyrolysates
of presently studiedChuaria from India rather supports an algal affinity. Moreover, the reflectance ofC circularis can be used to obtain a comparative maturity parameter of the Precambrian sediments. The review of the age and geographical
distribution ofC circularis constrains that this species cannot be considered as an index fossil for the Proterozoic time. 相似文献
2.
A well preserved assemblage of compressed, straight, circular to sinuously coiled megascopic and helical carbonaceous fossils and other varied megascopic morphoforms are known from the Early Mesoproterozoic Rohtas Formation, Semri Group within Vindhyan Supergroup exposed in Katni district of central India. These megascopic remains are preserved as impressions, compressions, partially mineralized remains, and/or epi-relief. Some of the forms are typical filamentous empty sheaths and others are trichomes, with cell like entities under various stages of degradation. This study, based on fresh collections and also of the topotype material of the helically coiled megascopic fossils, straight forms and related fossilized remains occurring as epi-relief from Katni indicate that the two morphotaxa are distinct entities and possibly appear to be prokaryotes. Grypania spiralis and Katnia singhii are most likely of cyanobacterial origin. Spirally coiled and circular fossils, with epi-relief, and which probably represents a tissue grade organism, are considered as Spiroichnus beerii Mathur, 1983. Linear sheet-like carbonaceous solitary form has been placed in the morphotaxon Proterotainia and described as P. katniensis n. sp. Certain rare circular, carbonaceous forms are considered as Chuaria sp. A few circular disc-like forms found in the assemblage are treated as dubiofossils. 相似文献
3.
Mukund Sharma 《Journal of Earth System Science》2006,115(1):67-98
Mesoproterozoic (∼ 1600 Ma old) Salkhan Limestone (Semri Group) of the Vindhyan Supergroup, exposed in Rohtas district of
Bihar, India, preserves an abundant and varied ancient microbial assemblage. These microfossils are recorded in three distinctly
occurring cherts viz., bedded chert, stromatolitic chert and cherty stromatolites. 27 morphoforms belonging to 14 genera and
21 species have been recognized. Six unnamed forms are also described.
The microbial assemblage, almost exclusively composed of the remnants of cyanobacteria, is dominated by entophysalidacean
members and short trichomes and can be termed as ‘typical Mesoproterozoic microbiotas’. The assemblage includes characteristic
mat-forming scytonematacean and entophysalidacean cyanobacteria.Eoentophysalis is the dominant organism in the assemblage. Ellipsoidal akinetes of nostocalean cyanobacteria(Archaeollipsoides) and spherical unicells also occur; both are distinct from mat forming assemblage, allochthonous and possibly planktic. Co-occurrence
of the microbiotas and precipitates is related to the depositional environment of the Mesoproterozoic tidal flats with high
carbonate saturation. 相似文献
4.
《Chemie der Erde / Geochemistry》2019,79(2):408-420
Chemico-mineralogical attributes of authigenic clays associated with the altered volcanic tuffs that occur in the Palaeoproterozoic Porcellanite Formation contain evidences of hydrothermal alteration and diagenetic processes in a marine environment. Previous sedimentological and geochemical studies on Porcellanite Formation were restricted to the Chopan area, but, the details related to provenance, nature and source of volcanism archived in these clays have not been ascertained. In order to understand these aspects, present study on these authigenic clays were carried out. Clay minerals represent dominance of illite with subordinate amount of montmorillonite. Moreover, low abundance of kaolinite is also noticed. The illite fibers and plates associated with the kaolinite indicate illitization. The kaolinite to illite transformation is favoured by incorporation of K+ ions, derived from the K-feldspar dissolution and its overgrowth. Major oxide contents of these clays and their ratios when plotted over diagrams marked with standard illite, kaolinite, smectite and chlorite compositional fields show clustering within or close to the illite field. Thermodynamic components calculated for these clays when plotted over AR23+AlSi3O10(OH)2 − R23+Si4O10(OH)2 − AR2+R3+Si4O10(OH)2 ternary diagram, data plots lie within the illite, mixed layer I/S and smectite fields. Binary major oxide data plots between bulk rock and authigenic clay compositions showed felsic affinity. Montmorillonite and illite predominated in the eastern and western marginal areas of the Vindhyan Basin, respectively. However, former resulted from the hydrothermal alteration of volcanic glass associated with the ferruginous breccia and altered tuffs and remnants of the volcanic vents, whereas, later is associated with the tuffaceous beds. Owing to the adsorption, Ba, Rb and Sr is enriched in clays comparing to the bulk rock composition. Low (< 15 ppm) Sc values suggested major contribution from the felsic component. Also, low Rb/Sr and Th/U values revealed moderate insitu weathering. The dominance of K-feldspar alteration and insitu weathering is also evident from clustering of clay data plots in the A-CN-K ternary diagram. Pronounced negative Eu anomaly together with higher LREE/HREE values associated with these clay minerals implied proximity to source and their possible derivation from the silicified felsic tuffs available in the provenance. 相似文献
5.
Jyotiranjan S Ray 《Journal of Earth System Science》2006,115(1):149-160
The Vindhyan Supergroup of India is one of the largest and thickest sedimentary successions of the world. Deposited in an
intra-cratonic basin, it is composed mostly of shallow marine deposits. It is believed to have recorded a substantial portion
of Proterozoic time and therefore, likely to contain valuable information on the evolution of the atmosphere, climate, and
life on our planet. It also contains some of the most disputed fossils of earliest animal life. Despite their importance,
the absolute age of these rocks had remained unknown until recently. In this work I evaluate all the recent chronological
information and discuss their implications. From the present findings it appears that the issues surrounding the age of the
Lower Vindhyans in the Son valley are now resolved, whereas problems with the age of the Upper Vindhyans and that with the
stratigraphic correlations remain to be answered. 相似文献
6.
Santanu Banerjee 《Gondwana Research》2000,3(4):521-528
The 1.2 Ga-old Koldaha shale, central India reveals three orders of depositional cyclicities in its basal storm-dominated shelf succession. Visual appraisal as well as Fourier and MEM analyses concurs in this respect. Only the major storm events at intervals of a few thousands of years have left recognizable imprints. Interbedding of storm sandstones and fairweather shales is apparently climate-controlled. Packaging of about seven such climatic cycles results the second-order cyclicity befitting eccentricity cycles of contemporary scale. Nonetheless, for the erratic storm bed-thickness trends within the cycles some other factor/s might have played a role. The third order cycles are, more dominantly, correlatable with basinal tectonics. 相似文献
7.
Mafic-ultramafic rocks of Archaean age constitute a significant component of the Eastern Indian Craton. These occur in two
different modes. In the eastern belt these occur as a long, linear enclave within the Singhbhum granite and the primary banding
in them is subvertical. In the more extensive western belt along the periphery of the Singhbhum granite, the disposition of
the primary banding is subhorizontal.
The major rock type in both the belts is meta-basalt with minor peridotitic komatiite and basaltic komatiite occurring in
the eastern belt. Rare ultramafic rocks with cumulate textures are present in both the belts. The larger volume of the basaltic
rocks preclude the possibility of their being derived by fractional crystallization of the high-MgO components.
On the basis of trace element and REE characters the rocks may be classified into three groups. One of the groups shows a
tholeiitic trend and include samples mostly from the eastern belt while the second consisting mostly of samples from the western
belt shows a calc-alkaline trend. The third group includes samples having elemental ratios intermediate between these two
groups. Zr/Nb ratios for the tholeiitic and calc-alkaline samples are different suggesting their sources to be different.
The tholeiitic samples have been generated from a source having chondritic REE characters, while the calc-alkaline samples
have been generated from a source with LREE enriched character. The high-MgO components in both the groups are suggested to
represent high degrees of melting compared to the basalts in each group.
It is further suggested that the tholeiitic basalts have been generated relatively early from a chondritic source. Down-buckling
of this material has added LREE enriched melts to the source, thereby changing its character into a LREE enriched one. Melting
of a source with such changed character has subsequently produced the calc-alkaline melts. Rocks with variable but intermediate
characters between these two groups have been generated as a result of contamination between these two groups. 相似文献
8.
The chemical composition of siliciclastics from Mesoproterozoic Kaimur Group, Vindhyan Supergroup have been investigated to
determine and appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature.
The studies pertaining to the relationship between major, trace and REE and the grain size variation were carried out along
Markundi ghat section, Sonbhadra district, Son valley. Upper Kaimur Group has been divided into three principal formations-the
uppermost-Dhandraul quartzites, Scarp sandstone and Bijaigarh shales. Petrographically, they range from quartz arenite, sublitharenite,
subfeldspathic arenite to pyritiferous shale. The elemental enrichment, except for SiO2, is in the following order-Bijaigarh Shale > Scarp Sandstone ≫ Dhandraul Sandstone. 相似文献
9.
Subir Sarkar Santanu Banerjee Pradip Samanta Silambuchelvan Jeevankumar 《Journal of Earth System Science》2006,115(1):49-60
This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-old Chorhat Sandstone of
the Semri Group — the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents
a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet.
The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic
because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation
potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack
of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between
the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion
and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be
smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands. 相似文献
10.
赣中存在中元古代变质基底的岩石地球化学证据及其地质意义 总被引:4,自引:0,他引:4
对江西乐安相山地区产出在石榴黑云母片岩内、原岩为拉班玄武质火山岩的斜长角闪岩进行了Sm-Nd同位素组成测定,获得Sm-Nd全岩等时线年龄为1113±49 Ma,结合弋阳梅树湾斜长角闪岩及余江马荃斜长角闪岩的同位素定年结果(1159±69 Ma,Sm-Nd; 1190±19Ma, U-Pb) 确认赣中变质岩带原岩属中元古代地层。采用微量元素地球化学比值模糊聚类分析方法对浙西陈蔡群、震旦系及赣中变质岩和震旦系地层进行对比,结合斜长角闪岩Pb-Nd同位素地球化学特征研究,确认赣中变质岩系与陈蔡群相当,从而为赣中变质岩归属于华夏地块变质基底提供了重要佐证。根据崇仁-临川-东乡晚白垩纪断陷红盆两侧深部地球物理重力场、磁场及地层和岩浆岩分布特征的对比,推断遂川深断裂为华夏地块在江西省境内的西北段边界。 相似文献
11.
The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleoproterozoic to the Neoproterozoic
period. Chronostratigraphic significance of stable carbon and oxygen isotope ratios of the carbonate phase in Vindhyan sediments
has been discussed in some recent studies. However, the subtle controls of facies variation, depositional setting and post-depositional
diagenesis on stable isotope compositions are not yet clearly understood. The Vindhyan Supergroup hosts four carbonate units,
exhibiting a wide variability in depositional processes and paleogeography. A detailed facies-specific carbon and oxygen isotope
study of the carbonate units was undertaken by us to investigate the effect of these processes and to identify the least altered
isotope values. It is seen that both carbon and oxygen isotope compositions have been affected by early meteoric water diagenesis.
The effect of diagenetic alteration is, however, more pronounced in case of oxygen isotopes than carbon isotopes. Stable isotope
compositions remained insensitive to facies only when sediments accumulated in a shallow shelf setting without being exposed.
Major alteration of original isotope ratios was observed in case of shallow marine carbonates, which became exposed to meteoric
fluids during early diagenetic stage. Duration of exposure possibly determined the magnitude of alteration and shift from
the original values. Moreover, dolomitization is found to be accompanied by appreciable alteration of isotope compositions
in some of the carbonates. The present study suggests that variations in sediment depositional settings, in particular the
possibility of subaerial exposure, need to be considered while extracting chronostratigraphic significance from δ13C data. 相似文献
12.
On the western and southern margins of the sickle shaped Vindhyan basin of north Indian shield, there are basal Vindhyan mafic volcanic rocks referred to as Khairmalia volcanics and Jungel volcanics respectively. These volcanics vary in composition from low-Ti tholeiite to high-Ti alkali basalt showing close affinity with continental flood basalts (CFB) and ocean island basalts (OIB) respectively. The parental magmas of Khairmalia and Jungel alkali basalts were formed by different degrees of partial melting of a garnet lherzolite. The magma of Khairmalia tholeiites was generated by a relatively higher degrees of partial melting of a garnet + spinel lherzolite. The geochemical data coupled with available geological and geophysical data favour a rift type origin of this basin which evolved as a peripheral basin showing many similarities with Paleogene Himalayan foreland basin. The existing radiometric age data suggest that the origin of Vindhyan basin is linked with Aravalli–Satpura orogeny. At about 1800–1600 Ma collision occurred along the Aravalli-Delhi fold belt (ADFB) and Central Indian Tectonic Zone (CITZ) with west and south subduction respectively. During this process the subducting lithosphere suffered extensional deformation on its convex side and some pre-existing large faults in the already thin leading edge of subducted plate also reactivated and tapped magma generated by decompressional melting of the subcontinental mantle. The simultaneous processes such as flexural subsidence, reactivation of pre-existing faults, heating, thermal cooling and contraction during volcanism, resulted in the formation of curvilinear warp parallel to the emerging mountain front. The Lower Vindhyan volcano–sedimentary succession was deformed and exposed to erosion before the deposition of Upper Vindhyan rocks. The orogenic forces were active intermittently throughout the Vindhyan sedimentation. 相似文献
13.
The Marwar Supergroup refers to a 1000–2000 m thick marine and coastal sequence that covers a vast area of Rajasthan in NW–India. The Marwar Basin uncomformably overlies the ∼750–770 Ma rocks of the Malani Igneous Suite and is therefore considered Late Neoproterozoic to Early Cambrian in age. Upper Vindhyan basinal sediments (Bhander and Rewa Groups), exposed in the east and separated by the Aravalli–Delhi Fold Belt, have long been assumed to coeval with the Marwar Supergroup. Recent studies based on detrital zircon populations of the Marwar and Upper Vindhyan sequences show some similarity in the older populations, but the Vindhyan sequence shows no zircons younger than 1000 Ma whereas samples taken from the Marwar Basin show distinctly younger zircons. This observation led to speculation that the Upper Vindhyan and Marwar sequences did not develop coevally.While there are alternative explanations for why the two basins may differ in their detrital zircon populations, paleomagnetic studies may provide independent evidence for differences/similarities between the assumed coeval basins. We have collected samples in the Marwar Basin and present the paleomagnetic results. Previous paleomagnetic studies of Marwar basinal sediments were misinterpreted as being indistinguishable from the Upper Vindhyan sequence. The vast majority of our samples show directional characteristics similar to the previously published studies. We interpret these results to be a recent overprint. A small subset of hematite-bearing rocks from the Jodhpur Formation (basal Marwar) exhibit directional data (Dec = 89° Inc = −1° α95 = 9°) that are distinct from the Upper Vindhyan pole and may offer additional support for temporally distinct episodes of sedimentation in these proximal regions. A VGP based upon our directional data is reported at 1°S 344°E (dp = 5°, dm = 9°). We conclude that the Marwar Supergroup developed near the close of the Ediacaran Period and is part of a larger group of sedimentary basins that include the Huqf Supergroup (Oman), the Salt-Range (Pakistan), the Krol–Tal belt (Himalayas) and perhaps the Molo Supergroup (Madagascar). 相似文献
14.
The Palaeoproterozoic Aravalli Supergroup in Salumber region includes a basal unit of metabasic volcanic rocks (Salumber volcanic
rocks) overlain by a volcaniclastic/conglomerate one. Although these volcanic rocks have been metamorphosed to green-schist
facies, some primary volcanic features are still preserved. This metabasic volcanic sequence can be further differentiated
on the basis of textural variations, and the mineral assemblages are: (a) oligoclase + actinolite + chlorite + epidote; and
(b) oligoclase + hornblende+ chlorite + biotite + Fe-Ti oxides. The SiO2 content ranges from ∼47.7 to 55.8% and MgO from ∼4.2 to 12.8%. Geochemical characteristics allow their subdivision into high
Mg and Fe tholeiites. Inverse relationship of MgO with silica, alkalis and Zr is generally consistent with fractionation mechanism,
also suggested by a change in colour of the rocks from dark greenish to light greenish towards the upper parts of the sequence.
These metabasic volcanic rocks are enriched in incompatible trace elements and LREE (La = 30 − 40 × chondrite, Lu = 2 − 5
× chondrite), and demonstrate affinity mainly with MORB and within plate settings in geochemical tectonic discrimination schemes.
The geochemical characteristics suggest a complex evolutionary history envisaging derivation of the melt from an enriched
heterogeneous lithospheric source. 相似文献
15.
Three major igneous events, dated at ~1465, ~1070 and ~500 Ma, are represented in the Proterozoic of central Western Australia, yet their extent is poorly understood. The compositions of dated mafic rocks from the western Bangemall Supergroup of Western Australia have been used to establish a chemical fingerprint for the ~1465 Ma and ~1070 Ma intrusive events, and to assign sills of an unknown age to one of the two events. A similar approach has been used to identify the extent of ~500 Ma magmatism. Low-pressure fractionation or accumulation has exerted a strong influence on the chemistry of rocks from all magmatic events, but distinctive trace-element ratios (e.g. Th/Nb, Nb/Zr) and rare-earth element (REE) chemistry (e.g. Eu/Eu?, (Gd/Yb)CN) can discriminate different events. These ratios remain constant regardless of the degree of fractionation or accumulation, and reflect the chemistry of the respective mantle sources. Based on chemistry, ~500 Ma igneous rocks are not found in the Bangemall Supergroup. Neodymium model ages for ~1465 Ma sills overlap with crystallisation ages of subduction-related felsic intrusive rocks from the adjacent Gascoyne Complex, and this, combined with trace-element and REE chemistry, suggests that the mantle source for these sills underwent ~5% crustal contamination approximately 450 Ma prior to melting in a subduction zone environment. Unrealistically large amounts of contaminant are required to explain the chemistry of most ~1070 Ma sills, and their chemistry is better explained by melting of a heterogeneous mantle source, consistent with the overlap of Nd model ages and crystallisation ages. However, the relatively low εNd(T) and high 87Sr/86Sr(T), elevated light REE, Rb and Zr concentrations, and high Th/Nb of some ~1070 Ma eastern Bangemall Supergroup sills are indicative of crustal contamination. These sills are relatively depleted in chalcophile elements and platinum-group elements, consistent with coeval precipitation of sulfides and crustal contamination. The overlap in the maximum depositional age of host-rocks with the crystallisation age of some sills, the confining of most ~1465 Ma sills to older parts of the stratigraphy, and field evidence showing that some sills belonging to both the ~1465 and ~1070 Ma events were intruded into wet sediments, indicate a close temporal relationship between sedimentation and sill intrusion. 相似文献
16.
N. V. Chalapathi Rao 《Journal of Earth System Science》2006,115(1):161-183
Amongst all the perceptible igneous manifestations (volcanic tuffs and agglomerates, minor rhyolitic flows and andesites,
dolerite dykes and sills near the basin margins, etc.) in the Vindhyan basin, the two Mesoproterozoic diamondiferous ultramafic
pipes intruding the Kaimur Group of sediments at Majhgawan and Hinota in the Panna area are not only the most conspicuous
but also well-known and have relatively deeper mantle origin. Hence, these pipes constitute the only yet available ‘direct’
mantle samples from this region and their petrology, geochemistry and isotope systematics are of profound significance in
understanding the nature of the sub-continental lithospheric mantle beneath the Vindhyan basin. Their emplacement age (∼ 1100
Ma) also constitutes the only reliable minimum age constrain on the Lower Vindhyan Group of rocks. The Majhgawan and Hinota
pipes share the petrological, geochemical and isotope characteristics of kimberlite, orangeite (Group II kimberlite) and lamproite
and hence are recognised as belonging to a ‘transitional kimberlite-orangeite-lamproite’ rock type. The namemajhagwanite has been proposed by this author to distinguish them from other primary diamond source rocks. The parent magma of the Majhgawan
and Hinota pipes is envisaged to have been derived by very small (<1%) degrees of partial melting of a phlogopite-garnet lherzolite
source (rich in titanium and barium) that has been previously subjected to an episode of initial depletion (extensive melting
during continent formation) and subsequent metasomatism (enrichment). There is absence of any subduction-related characteristics,
such as large negative anomalies at Ta and Nb, and therefore, the source enrichment (metasomatism) of both these pipes is
attributed to the volatile- and K-rich, extremely low-viscosity melts that leak continuously to semi-continuously from the
asthenosphere and accumulate in the overlying lithosphere. Lithospheric/crustal extension, rather than decompression melting
induced by a mantle plume, is favoured as the cause of melting of the source regions of Majhgawan and Hinota pipes. This paper
is a review of the critical evaluation of the published work on these pipes based on contemporary knowledge derived from similar
occurrences elsewhere. 相似文献
17.
18.
Petrological and geochemical studies have been carried out on Pulivendla and Gandikota Quartzite from Chitravati Group of Cuddapah Supergroup to decipher the provenance and depositional environment. Both the units are texturally mature with sub-rounded to well-rounded and moderately to well-sorted grains. Majority of the framework grains are quartz, in the form of monocrystalline quartz, followed by feldspars (K-feldspar and plagioclase), mica, rock fragments, heavy minerals, with minor proportion of the matrix and cement. Based on major element geochemical classification diagram, Pulivendla Quartzite is considered as quartz-arenite and arkose to sub-arkose, whereas Gandikota Quartzite falls in the field of lith-arenite and arkose to sub-arkose. Weathering indices like CIA, PIA, CIW, ICV, Th/U ratio and A–CN–K ternary diagram suggest moderate to intense chemical weathering of the source rocks of these quartzites. Whole rock geochemistry of quartzites indicate that they are primarily from the first-cycle sediments, along with some minor recycled components. Also their sources were mostly intermediate-felsic igneous rocks of Archean age. The tectonic discrimination plots, Th–Sc–Zr/10 of both these formations reflect active to passive continental margin setting. Chondrite-normalized rare earth element (REE) patterns, and various trace element ratios like Cr/Th, Th/Co, La/Sc and Th/Cr indicate dominantly felsic source with minor contribution from mafic source. Th/Sc ratios of Pulivendla and Gandikota Quartzite are in close proximity with average values of 2.83, 3.45 respectively, which is higher than AUCC (\(\hbox {Th/Sc}=0.97\)), demonstrating that the contributions from more alkali source rocks than those that contributed to AUCC. 相似文献
19.
Meenal Mishra Vaibhava Srivastava P. K. Sinha Hari B. Srivastava 《Journal of the Geological Society of India》2017,89(4):375-385
Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intrabasinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin. 相似文献
20.
The Central Tianshan belt in northwestern China is a small Precambrian block located in the southern part of the Central Asia Orogenic Belt (CAOB), which is considered as "the most voluminous block of young continental crust in the world" that comprises numerous small continental blocks separated by Paleozoic magmatic arcs. The Precambrian basement of the central Tianshan Mountains is composed of volcanic rocks and associated volcano-sedimentary rocks that were intruded by granitic plutons. Geochemical analyses demonstrate that the granitic plutons and volcanic rocks were generated in the Andean-type active continental arc environment like today's Chile, and the zircon U-Pb SHRIMP dating indicates that they were developed at about 956 Ma, possibly corresponding to the subduction of the inferred Mozambique Ocean under the Baltic-African super-continent. 相似文献