Cambrian (similar to 510 Ma) ophiolites of the East Kunlun orogen, China: A case study from the Acite ophiolitic tectonic melange

摘要

The Acite ophiolitic melange represents a remnant of the Proto-Tethys forearc oceanic lithosphere. Two gabbros yield zircon Pb-206/U-238 ages of 510-512 Ma. The magmatic rocks are divided into three subtypes: (1) Mid-ocean ridge basalt (MORB) -like forearc basalts (FABs), (2) Low-titanium tholeiitic gabbros (LTGs), and (3) Normal calc-alkaline dacites (CADs). The FABs have comparatively higher TiO2 concentrations (1.13-1.42 wt%) and show almost flat REE patterns. In the NMORB normalized trace element patterns, the FABs display flat distributions of high field strength element (HFSE). The epsilon(Nd)(t) values of FABs range from +4.0 to +4.8. These features are similar to the composition of Izu-Bonin-Mariana forearc basalts (FABs). LTGs are characterized by higher contents of MgO (8.85-9.95 wt%) and lower concentration of TiO2 (0.29-0.50 wt%). They show LREE-depleted patterns, however having comparatively lower total REE contents than those of FABs. The eNd(t) values of LTGs range from +7.4 to +8.4. These features show that LTG magmas originated from a progressively depleted mantle source. In contrast, the CADs having higher SiO2 contents (63.58-70.92 wt%) have higher total REE contents. In the NMORB normalized trace element patterns, CADs are characterized by enrichment of LILEs and depletion of HFSEs. CADs have negative epsilon(Nd)(t) values ranging from -9.8 to -10.5, which are likely suggestive of a crust-derived source. The rock association of FABs and LTGs, together with tectonic discrimination plots and regional data, suggest a forearc setting above the SSZ for Acite ophiolitic melange.