Whole rock O-isotopes of the Panjal Traps are presented in order to assess the influence of crustal contamination and hydrothermal alteration in their genesis. The basalts from the eastern Kashmir Valley and Lidder Valley have enriched δ18OVSMOW values (9.0‰ to 12.0‰). The basalt with the lowest δ18OVSMOW values (9.0‰ and 9.2‰) were likely affected by deuteric alteration but their values could be close to the original melt composition as the rocks do not show trace element (Th/NbPM ≈0.8; Nb/U ≈ 50; Th/La ≈0.1) or isotopic evidence of crustal contamination (87Sr/86Sri = 0.7043 to 0.7045; εNd(t) = +1.1 to +1.3). The δ18OVSMOW (> 12‰) values and Nd isotopes (εNd(t) < −8.6) of the silicic Panjal Traps are consistent with derivation from continental crust. The remaining mafic rocks have enriched Sr-Nd-O isotopic values that indicate crustal (10–30%) contamination (εNd(t) = −1.9 to −6.1; 87Sr/86Sri = 0.7051 to 0.7087; δ18O = 10.1‰ to 12.0‰) with the upper flows exhibiting further enrichment by hydrothermal alteration. The basalts from the Pir Panjal Range, western Kashmir Valley, have variable Nd isotopic values (εNd(t) = −6.8 to +4.3) and the lowest δ18OVSMOW values (6.8‰ to 7.9‰) of the study. The results demonstrate that the rocks from the Pir Panjal Range preserved not only differences in radiogenic isotopes but also the O-isotopes as well. The change in the Nd and O isotopes of the basalt from Guryal Ravine and Lidder Valley to the more depleted values of the Pir Panjal Range is likely due to a transition from a chondritic mantle source to a depleted mantle source as the continental rift evolved to a sea-floor spreading environment.
Abstract Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo92.5–88.9), orthopyroxene (Mg# = 94.6–89.2), and clinopyroxene (Wo49.1–38.1En57.0–45.4Fs3.0–11.0) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# < 45) and low-Al (Cr# > 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95–2.07 wt %; Mg# = 88.5–90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (87Sr/86Sri = 0.704400–0.707908; εNd(t) = 0.0 − +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 °C with the majority of temperature estimates >1000 °C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded ΔFMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1–2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya <1000 °C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (<6 Ma) between the Luzon arc and the Eurasian margin.
The Neoarchean Sonakhan greenstone belt (SGB) in the north‐eastern Bastar Craton is mainly composed of an association of pillowed and massive tholeiitic basalts emplaced within an intraoceanic environment. Two types of ultrabasic rocks were identified and are designated as TH‐1 and TH‐2 on the basis of geochemical parameters. Both sample suites exhibit depletion of HFSE with reference to LILE and LREE, LREE/HFSE ratios, and negative Nb–Ta–Ti anomalies in the primitive mantle‐normalized multi‐element diagrams. The geochemical characters of TH‐1 and TH‐2 are consistent with Island arc tholeiites (IAT) and boninite‐like rocks of both Archean and Phanerozoic terranes. Mineral chemistry of clinopyroxenes from both volcanic suites indicate an oceanic arc affinity. The chromite chemistry indicate its derivation from a boninite‐like magma in a suprasubduction zone (SSZ) environment. Trace element modelling depicts that source depletion with significant influence of fluids derived from the subducting oceanic slab collectively controlled the trace element inventory of the mantle wedge, which result to the higher abundance of LREE and LILE compared to HREE and HFSE. The occurrence of lower basalt sequence followed by IAT and boninite‐like rocks in the SGB define a magmatic evolution which is comparable to the Phanerozoic ophiolite suites that exhibit subduction initiation prior to forearc rifting and suprasubduction zone magmatism. The subduction initiation in the intraoceanic lithosphere and the initial decompression melting followed by the upwelling of the MORB‐related magmas were attributed to the formation of the lower pillow basalts. Dehydration of the subducting plate and the melt extraction processes in the hydrous lherzolite mantle wedge account for the formation of TH‐1. Subsequent depletion and significant modification of a more refractory harzburgite source by the slab‐derived fluids generated the TH‐2 rocks. The compositionally diverse magmatic imprints with IAT and boninite affinity in the SGB indicate the significance of episodic melt extraction processes, dehydration of the subducting plate, and hydrous fluxing in SSZ tectonic setting.
The Sonakhan greenstone belt (SGB), located in the north‐eastern Bastar craton of the Indian shield, comprises mafic‐ultramafic and volcanic‐intrusive sequences in the lower stratigraphic units. We investigate the Platinum‐group element (PGE) relations of Boradih intrusion of the SGB to evaluate its tectono‐magmatic evolution. The chondrite‐normalized PGE patterns of boninitic cumulate rocks exhibit higher abundance of Palladium group PGEs (∑PPGE = 292–496 ppb) relative to the Iridium group PGEs (∑IPGE = 32–52 ppb) along with variable Au concentrations (51.34–718.05 ppb). The PGE concentrations are attributed to a boninitic parental melt, where the IPGEs in the source possibly partitioned into a monosulphide solid solution. The Cu (22–80 ppm), elevated Pt (22–238 ppb), and Pd (31–377 ppb) concentrations indicate Pt and Pd have partitioned into a semi‐metal rich melt during the later stages of crystallization. The geochemical characteristics of the basalts and ultramafic cumulates of the SGB indicate a supra‐subduction zone tectonic setting for its formation. Similar geochemical and litho‐tectonic correlations are also noticed between the SGB of Bastar craton and greenstone belts of the Eastern Dharwar craton of south India. The SGB (V/Yb =146 ± 25) and greenstone belts of Eastern Dharwar craton (V/Yb = 134 ± 52) record similar oxidation conditions of Phanerozoic subduction zones. Accordingly, we propose magmatic as well as tectonic correlations are possible for the Archean‐Palaeoproterozoic Bastar and Eastern Dharwar cratons.
The Deccan large igneous province (DLIP) of Peninsular India is predominantly composed of tholeiitic basalts with a minor amount of alkaline, carbonatite, and silicic rocks. The tholeiitic basalts of DLIP are enriched in incompatible trace elements and divisible into low‐Ti basalts (TiO 2 < 2.5; Ti/Y < 500) and high‐Ti basalts (TiO 2 > 2.5; Ti/Y > 500). Despite variable total REE content, both low‐Ti and high‐Ti basalts exhibit almost similar geochemical patterns in the chondrite‐ and primitive‐ mantle‐normalized diagrams. The mantle potential temperature (T p ) of high‐Ti tholeiites estimated at Ca.1368°C, which is consistent with an ambient mantle temperature (1300–1400°C) whereas the T p of low‐Ti tholeiites is around 1553°C, which indicates its derivation from the mantle plume axis. Extensive assimilation of the crustal components into the ascending plume have masked the nature and compositional characteristics of both high‐ and low‐Ti basalts. The low‐Ti basalts indicate significant contamination with crustal components than the high‐Ti basalts. The geochemical and isotopic compositions indicate the involvement of multiple mantle components. In general, the differentiated tholeiites of DLIP are the products of plume and depleted sections of the mantle with or without contributions from the continental crust. The main phase of Deccan magmatism intensely affected the atmosphere and hydrosphere, which was a possible trigger for the mass extinction event at the K–Pg boundary.
The Neoarchean Sonakhan Greenstone Belt, located in the northeastern fringes of Bastar Craton, Central India, is dominated by basalts, andesites, dacites, and rhyolites and also contains some basic rocks with very high MgO (up to 33.4 wt%). Chromite mineralization is present in these rocks along with the cumulates of olivine and clinopyoxenes. The rocks are classified as siliceous high‐magnesium basalts (SHMB) exhibiting enriched large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements. Elevated Th/Yb ratios and negative Nb‐Ta‐Ti anomalies in the primitive mantle normalized multielement diagram indicates a significant role of subduction‐related melts/fluids in their genesis. The chromites in SHMB have high Cr# (0.67–0.75) and moderate Mg# (0.11–0.5) values. Parental melt calculations in the chromites indicate that they are crystallized from an SHMB magma in an island‐arc setting. A plausible model for the genesis of the rocks of Sonakhan Greenstone Belt includes initial subduction of an intraoceanic lithosphere followed by eruption of lava in an oceanic environment. Continued subduction of the slab followed by slab rollback followed by the generation of SHMB parental magma, which was introduced into the basal portions of the lithosphere in which cumulates of olivine and clinopyroxene have been developed and the chromite mineralization occurred in the inter cumulus space. Final emplacement of the magma took place in a forearc suprasubduction‐zone environment with SHMB signature carrying the cumulates, which were located in the lower part of the lithosphere.
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