Chromites from the Nidar ophiolite and Karzok complex, Transhimalaya, eastern Ladakh: their magmatic evolution

摘要

In the Indus Suture Zone, Ladakh Transhimalaya, chromites occur in different modes, viz. (a) as disseminations (chromite N1) and massive chromitite (chromite NIC) in dunite of the Nidar ophiolite and (b) as disseminations (chromite NIC2) in metamorphosed gabbro of the Karzok complex. Chromites occur both as cumulus and intercumulus phases and show a resorbed boundary with olivine. XRD study shows their uniform lattice parameters (a_0 ～ 8.33 A). EPMA scan shows variable Cr_2O_3 content of chromite N1 ranging from 52.7 to 60.4% and restricted Cr_2O_3 content for chromite NIC as 61.0-61.5%. The chromite NIC2 also shows a narrow range of Cr_2O_3 content (56.0-58.2%). A ~(57)Fe Moessbauer spectroscopic investigation shows that chromites have similar iron distribution as: Fe~(3+)(A), Fe~(2+)(A) and Fe~(2+)(B). The iron distribution have been linked to oxidation; the higher Fe~(3+)/Fe~(2+) ratio of chromite NIC (0.67) than that of chromite NIC2 (0.30) suggests higher oxidation of the former than the latter. High Mg-olivine (～Fo_(95)), high Cr-chromite (Cr# = 80) and Al_2O_3 wt% (12.01-12.22) of chromites from the Nidar ophiolite are typical for a boninitic parentage. The Cr# vs Mg# plot and FeO/MgO in melt (～ 0.5) also suggest derivation from the boninitic melt. On the other hand, the chemical signatures of chromite NIC2, Al_2O_3 wt% (14.91-15.21) and FeO/MgO (1.17-1.32) for corresponding melt are very different from a boninitic parentage and may be indicative of a depleted peridotite/MORB type-source. The partitioning of Mg and Fe between coexisting olivine and Cr-spinel in chromite NIC indicates sub-solidus temperatures of around 1000℃. The differences in chemical composition of chromites as well as parental melts suggest different sources for the Nidar ophiolite and the Karzok complex.