Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

The supra-subduction zone ophiolite or ophiolitic mélange formed in the forearc setting is generally considered to be a key geological record for subduction initiation (SI) with petrological characteristics comparable to the SI-related rock sequence from forearc basalt (FAB) to boninite in the Izu-Bonin-Mariana subduction zone. Nevertheless, the standard FAB and boninite are generally difficult to observe in the forearc rocks generated during SI. Yet, a typical rock sequence indicating the SI of the western Proto-Tethys Ocean is reported for the first time in the Qimanyute intra-oceanic forearc system in the western Kunlun Orogen, Northwest Tibetan Plateau. The magmatic compositions, which range from less to more high field strength element (HFSE)-depleted and large ion lithophile element (LILE)-enriched, are changing from oceanic plagiogranites (ca. 494 Ma) to forearc basalt-like gabbros (FAB-Gs, ca. 487 Ma), boninites, and subsequent Nb-enriched gabbros (NEGs, ca. 485 Ma), which are thus consistent with the Izu-Bonin-Mariana forearc rocks as well as the Troodos and Semail supra-subduction zone-type ophiolites. The geochemical data from the chemostratigraphic succession indicate a subduction initiation process from a depleted mid-oceanic-ridge (MORB)-type mantle source with no detectable subduction input to gradual increasing involvement of subduction-derived materials (fluid/melts and sediments). The new petrological, geochemical, and geochronological data, combined with the regional geology, indicate that the well-sustained FAB-like intrusive magmas with associated boninites could provide crucial evidence for SI and further reveal that the SI of the western Proto-Tethys Ocean occurred in the Late Cambrian (494−485 Ma).

Supplemental Material: Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Detailed petrological and geochemical data.

Supplemental Material: Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Detailed petrological and geochemical data.

Chemical Evolution of Nb-Ta Oxides and Cassiterite in Phosphorus-Rich Albite-Spodumene Pegmatites in the Kangxiwa–Dahongliutan Pegmatite Field, Western Kunlun Orogen, China

The Kangxiwa–Dahongliutan pegmatite field in the Western Kunlun Orogen, China contains numerous granitic pegmatites around a large granitic pluton (the Dahongliutan Granite with an age of ca. 220 to 217 Ma), mainly including barren garnet-, tourmaline-bearing pegmatites, Be-rich beryl-muscovite pegmatites, and Li-, P-rich albite-spodumene pegmatites. The textures, major element contents, and trace element concentrations of columbite-group minerals (CGM) and cassiterite from three albite-spodumene pegmatites in the region were investigated using a combination of optical microscopy, SEM, EPMA and LA-ICP-MS. The CGM can be broadly classified into four types: (1) inclusions in cassiterite; (2) euhedral to subhedral crystals (commonly exhibiting oscillatory and/or sector zoning and coexisting with magmatic cassiterite); (3) anhedral aggregates; (4) tantalite-(Fe)-ferrowodginite (FeSnTa2O8) intergrowths. The compositional variations of CGM and cassiterite are investigated on the mineral scale, in individual pegmatites and within the pegmatite group. The evolution of the pegmatites is also discussed. The variation of Nb/Ta and Zr/Hf ratios of the cassiterite mimics the Nb-Ta and Zr-Hf fractionation trends in many LCT pegmatites, indicating that these two ratios of cassiterite may bear meanings regarding the pegmatite evolution.