This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton(WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB(mid-ocean ridge basalt) type basalts(BTB) and the other corresponding to calc-alkaline andesites(BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin(BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N-to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB(back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-lik
One of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a suprasubduction zone(SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range(MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing(Moseley and Abbotts 1979, Dilek and Furnes, 2011;Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well-developed ophiolite, known as the Masirah ophiolite(Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains(Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of midCretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc-alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back-arc basin model for this ophiolite(e.g. Tamura and Arai, 2006;Godard et al., 2008;Rollinson and Adetunji, 2015). The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantlenormalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment. Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean(MOR) or suprasubduccion zone(SSZ)(Arai and Matsukage, 1998;
The Early Cretaceous Xigaze ophiolites(XO)exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet,preserved the structure of the upper mantle and oceanic crust,is interpreted as a definite record of Neo-Tethys oceanic lithosphere.Many evolution models of the XO have been proposed since the 1980s(Nicolas et al.,1981;Hébert et al.,2012;Maffione et al.,2015).However,the geodynamic environment of the XO whether at a MOR or SSZ remains controversial.As key evidence for the identification of the SSZ ophiolite,the boninite-like dolerites from the XO are still controversial and poorly constraint(Chen et al.,2003;Bao et al.,2013;Dai et al.,2013).According to previous reports,dolerites were subdivided to MORB-like type and boninite-like type(Chen et al.,2003;Dai et al.,2013).Apart from high-SiO2(>52 wt%),high MgO(>8 wt%)and low-TiO2(<0.5 wt%),we found the boninite-like and MORB-like dolerites are indistinguishable in outcrop,mineral and chemical.They are sills invaded into mantle sequence of the XO,with 50.35–56.80 wt%SiO2,0.32–1.19 wt%TiO2,7.39–8.89 wt%Fe2O3T,5.50–10.42 wt%MgO and Mg#of 0.49–0.74.MORB-like dolerites display trace element and REE patterns similar to those of most fore-arc or/and back-arc basalts from Izu-Bonin-Mariana(IBM),i.e.enrichment in large ion lithophile elements(LILEs,e.g.Cs,Rb,Ba,Pb,U)and depletion in high-field strength elements(HFSE,Nb,Ta)(Fig.1).Boninite-like dolerites resemble the MORB-like dolerites in trace elements and REE patterns,excepting for lower concentrations in REE,but distinct from the boninites discovered from the IBM or Troodos ophiolite(Fig.1c,d).Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs.Nb/Yb diagram(Fig.2a),suggesting constant subduction component added to the mantle source(Pearce et al.,1995).We chose depleted MORB source Mantle(DMM,Workman and Hart,2005)as the starting composition computing immobile element contents for primary melts using the methods and partition coe
