Oxidation states within the planetary interior are intrinsically linked with the broad scale tectonism; however, it is difficult to estimate the actual oxidation conditions. Orthopyroxene–magnetite symplectite formed by olivine oxidation may provide a significant clue into such oxidation events. Here we report detailed mineralogical and petrological synthesis of such orthopyroxene–magnetite symplectites from olivine gabbros of Oman Ophiolite (Hole GT2A, ICDP Oman Drilling Project). In order to understand how oxidation affects different olivine compositions, we employed a phase equilibria approach and computed several temperature–composition diagrams at a fixed pressure (1 kbar). Our experiments predict the coexistence of olivine with Fo75–76 and Fo71 with the orthopyroxene (En79 and En76), respectively, which is remarkably similar to the mineral chemistry obtained from the Oman lower crustal gabbros. From the magnetite content, we also infer that the symplectite formation may have taken place over a range of temperatures (600–1000 °C) via subsolidus olivine oxidation and/or melt (oxidizing)–olivine interaction. The latter is more probable, considering the partial occurrence of orthopyroxene and clinopyroxene rim adjacent to the symplectites.
The Andaman and Nicobar ophiolites, in the forearc of the western Sunda subduction zone, underwent enigmatic, rapid Cenozoic vertical motions: shallow-water sediments with abundant arc debris characterize the middle Paleocene–middle Eocene and are under- and overlain by significantly deeper sediments. Recent paleomagnetic results revealed a near-equatorial paleolatitude of the West Burma Block and the associated subduction zone, at a similar latitude as the Andaman forearc until the early Eocene, providing a new avenue toward explaining the unusual stratigraphy. Here, we studied the provenance of the clastic sediments of the Andaman-Nicobar accretionary ridge using petrography, geochemistry, and detrital zircon geochronology. We found that the Paleocene-Eocene Namunagarh Grit is likely to be derived from a then proximal, 60 Ma old arc that was likely located in the ocean to the north (present-day east) of the West Burma Block, west of Andaman-Nicobar. The Oligocene–lower Miocene East Andaman Flysch contains West Burma Block debris that traveled much farther and mixed with sediments derived from Sundaland. The West Andaman and Great Nicobar Flysch have an additional Himalayan source consistent with derivation from the downgoing plate. We interpret this history as reflecting the late Paleocene–early Eocene collision of the West Burma Block, likely then part of the Australian Plate, with the Andaman forearc causing uplift and proximal sedimentation shed from the colliding arc. Subsequent northward motion of the West Burma Block caused subsidence of the Andaman forarc and N-S opening of the Andaman Sea, which opened a pathway for Sundaland-derived sediments to reach the Andaman ophiolites. The recently proposed high Cenozoic mobility of the West Burma Block remains to be reconciled in detail with geological observations in Myanmar and Sundaland, but our results show that this scenario provides ample opportunity to explain the previously enigmatic stratigraphic evolution of the Andaman and Nicobar Islands.
This study aims to investigate the earliest imprint of Deccan rift magmatism as preserved in alkali basalts from the northwestern Indian shield. The alkali basalts are petrographically classified as nephelinites and basanites. They are silica undersaturated and their high Mg numbers, CaO/Al 2 O 3 , Cr and Ni indicate their primitive character. Geochemically, they are similar to global ocean island basalts; their bulk rock trace elements and Sr–Nd–Hf isotopic signatures suggest their derivation from a garnet-bearing peridotite field. However, their elevated values of Sr/Sm, Sm/Hf, Zr/Hf, Nb/Ta; positive Ba, Sr and negative Zr, Hf spikes suggest that the magma source represents a mixture of garnet peridotites and carbonated melts. Estimated primary melt compositions closely follow the trajectory defined by the high-pressure experimental partial melting trend of a low-carbonated peridotite source. The melting environment approximates to a high mantle potential temperature. A low 87 Sr/ 86 Sr ratio and a negative correlation between 176 Hf/ 177 Hf and 143 Nd/ 144 Nd of the alkali basalts suggest that the mantle source is a mixture of a depleted Indian MORB-type mantle and an enriched mantle type 2. We correlate this event with the melting of the leading edge of the Réunion plume head during Gondwana break-up in a relatively short span of the Cretaceous/Paleogene boundary.
Data and Supporting Information for the manuscript titled From back-arc to forearc: a geochemical and geochronological record of arc polarity reversal from the Andaman Ophiolite, SE Asia Contents: Table captions (Data and supporting information)
Table 1. Geographic location and petrographic summary of the studied samples of Andaman ophiolites Table 2. Bulk rock geochemistry of selected samples of volcanic rocks from Andaman ophiolites Table 3. U-Pb data for zircon from Andaman samples Supplementary Table 1. Analyses for blanks, replicates and standards used for bulk rock analysis of volcanic rocks Supplementary Table 2. Compilation of the available bulk rock geochemical data of volcanic rocks from Andaman and Nicobar Islands (including the data obtained in the present study) Figure captions (Supporting information)
Supplementary figure 1 Typical morphologies of zircon in the three populations investigated. Numbers correspond to those in Table 3. The grains are about 70-150 micron long.
Supplementary figure 2 N-MORB (a,d,g,j,m,p); with emphasis on LILE (b,e,h,k,n,q) and chondrite (c,f,i,l,o,r) normalized spidergram for volcanic rocks of Andaman ophiolite.
Supplementary figure 3 (a,b,c,d) primitive mantle; (e,f,g,h) chondrite normalized spidergram for clinopyroxene trace element concentration of forearc peridotites from literatures. (i,j) Domain of forearc peridotite with clinopyroxene data from Daito ridge (Morishita et al. 2018b). Clinopyroxene trace element pattern of Rutland Island peridotite is also shown. Supplementary figure 4 (a) AFM (Irvine and Baragar, 1971) plot and trace element-based alkalinity test diagram (Ross and Bédard, 2009) (b) Th/Yb-Zr/Y; (c) Y-Zr; (d) Yb-La, (e) Yb-Th for volcanic rocks of Andaman ophiolite Note: References in supporting information are cited in the main text and included in the reference list of the main paper. All data including supporting information for the manuscript are available at Figshare https://doi.org/10.6084/m9.figshare.11396469.v4
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