Multistage origin of dunite in the Purang ophiolite, southern Tibet, documented by composition, exsolution and Li isotope characteristics of constituent minerals

Abstract

The Purang ophiolite, which crops out over an area of about 650&kmspan classCombining double low line"inline-formula"2/span in the western Yarlung-Zangbo suture zone, consists chiefly of mantle peridotite, pyroxenite and gabbro. The mantle peridotite is comprised mainly harzburgite and minor dunite. Locally, the latter contains small pods of chromitite. Pyroxenite and gabbro occur as veins of variable size within the peridotite; most of them strike northwest, parallel to the main structure of the ophiolite./p pThree types of dunite occur in the Purang ophiolite: dunite that envelopes podiform chromitite (1) and lenses of dunite with either Cr-rich spinel (2) or Cr-poor spinel (3) in a harzburgite host. The constituent minerals of dunite envelopes around podiform chromitite are similar in composition to those of transition-zone dunite (span classCombining double low line"inline-formula"Fo91.01-91.87/span in olivine; span classCombining double low line"inline-formula"math xmlnsCombining double low line"http://www.w3.org/1998/Math/MathML" idCombining double low line"M3" displayCombining double low line"inline" overflowCombining double low line"scroll" dspmathCombining double low line"mathml"mrow classCombining double low line"chem"mi mathvariantCombining double low line"normal"Cr/mimo//momo(/momi mathvariantCombining double low line"normal"Cr/mimo+/momi mathvariantCombining double low line"normal"Al/mimo)/mo/mrow/mathspansvg:svg xmlns:svgCombining double low line"http://www.w3.org/2000/svg" widthCombining double low line"60pt" heightCombining double low line"14pt" classCombining double low line"svg-formula" dspmathCombining double low line"mathimg" md5hashCombining double low line"e31293309365b661f0139669d0a05c1f"svg:image xmlns:xlinkCombining double low line"http://www.w3.org/1999/xlink" xlink:hrefCombining double low line"ejm-32-187-2020-ie00001.svg" widthCombining double low line"60pt" heightCombining double low line"14pt" srcCombining double low line"ejm-32-187-2020-ie00001.png"//svg:svg/span/span (Cr#) span classCombining double low line"inline-formula"Combining double low line41.5/span-47.0 and span classCombining double low line"inline-formula"math xmlnsCombining double low line"http://www.w3.org/1998/Math/MathML" idCombining double low line"M5" displayCombining double low line"inline" overflowCombining double low line"scroll" dspmathCombining double low line"mathml"mrow classCombining double low line"chem"mi mathvariantCombining double low line"normal"Mg/mimo//momo(/momi mathvariantCombining double low line"normal"Mg/mimo+/momsupmi mathvariantCombining double low line"normal"Fe/mimrowmn mathvariantCombining double low line"normal"2/mnmo+/mo/mrow/msupmo)/mo/mrow/mathspansvg:svg xmlns:svgCombining double low line"http://www.w3.org/2000/svg" widthCombining double low line"80pt" heightCombining double low line"15pt" classCombining double low line"svg-formula" dspmathCombining double low line"mathimg" md5hashCombining double low line"eb5d0f7511af62deaabd00d6cf2a2c3b"svg:image xmlns:xlinkCombining double low line"http://www.w3.org/1999/xlink" xlink:hrefCombining double low line"ejm-32-187-2020-ie00002.svg" widthCombining double low line"80pt" heightCombining double low line"15pt" srcCombining double low line"ejm-32-187-2020-ie00002.png"//svg:svg/span/span (Mg#) span classCombining double low line"inline-formula"Combining double low line58.9/span-63.0 in Cr-spinel). Forsterite contents in olivine decrease from type 2 lenses with Cr-rich spinel (91.9-93.0) to type 1 dunite enveloping chromitite (91.7-93.7) to type 3 lenses with Cr-poor spinel (95.3-96.0). Similarly, Cr# in spinel decreases from type 2 (66.9-67.9) to type 1 (41.5-47.0) to type 3 (19.8-20.6). In addition, span classCombining double low line"inline-formula"Al2O3/span in clinopyroxene is highest in type 2 (3.48-5.24&wt&%) and decreases to type 1 (1.56-3.29&wt&%) and type 3 (0.78-0.86&wt&%). Olivine in type 1 dunite enveloping podiform chromitite has Li concentrations and span classCombining double low line"inline-formula"id/i7Li/span values of 1.48-1.71&ppm and 6.19&‰-7.98&‰, respectively. Type 2 dunite lenses with Cr-rich spinel contain olivine with Li span classCombining double low line"inline-formula"Combining double low line0.98/span-1.64&ppm and span classCombining double low line"inline-formula"id/i7Li/span span classCombining double low line"inline-formula"Combining double low line6.77/span&‰-10.99&‰. The type 3 dunite lenses with Cr-poor spinel show the highest values of Li span classCombining double low line"inline-formula"Combining double low line0.94/span-1.40&ppm and span classCombining double low line"inline-formula"id/i7Li/span span classCombining double low line"inline-formula"Combining double low line10.25/span&‰-14.20&‰. Exsolution lamellae of clinopyroxene and magnetite occur as oriented intergrowths in olivine of type 3 dunite lenses with Cr-poor spinel./p pWe suggest that the Purang ophiolite developed during two main stages of formation. In the first stage, abyssal peridotites formed in a mid-ocean-ridge environment. During the second stage, hydrous high-Mg boninitic melts were produced by high degrees of partial melting in a supra-subduction zone mantle wedge, which reacted with peridotite to form type 2 dunite pods with high-Cr# spinel. At lower degrees of partial melting in the same mantle wedge, Al-rich melts were produced, which reacted with peridotite to form type 3 dunite pods that contain low-Cr# spinel. These Al-rich melts were also relatively rich in span classCombining double low line"inline-formula"Ti4+/span, span classCombining double low line"inline-formula"Ca2+/span and span classCombining double low line"inline-formula"Fe3+/span, which were incorporated into the olivine structure by appropriate substitutions. During cooling, these elements exsolved as lamellae of magnetite and clinopyroxene.