Zircon U-Pb Age and Geological Implications of Ultramafic Rocks in Changshagou, Altun Area, Xinjiang Province

During 1:50000 scale regional survey, the author discovered that the ultramafic rocks from Altun ophiolite tectonic mixtite belt are mainly composed of plagioclase pyroxene amphibolite, pyroxene peridotite, and serpentinized peridotite. Geochemistry of the diabase shows that the geochemistry of the major is characterized by low contents of SiO₂, CaO, K₂O, Na₂O and high contents of MgO and FeO. With a low DI value (0.35-14.45), the magmatic differentiation is not obvious; while the solidification degree, with a high SI value (47.82-70.05), is relatively high. The ultramafic rocks belong to calcium series and magnesian ultrabasic rock, with the Rittman index of 0.97 and the M/F index from 1.94 to 4.39. Among the compositions, the TiO₂ content in pyroxene peridotite is close to that of mid-ocean ridge mantle(0.10%-0.40%). Characteristics of rare earth element and trace element are similar to those of mantle peridotite. All these characteristics indicate that the ultramafic rock is a part of ophiolite suite derived from the primitive mantle. The plagioclase pyroxene amphibolite may be ultramafic diamictite. Meanwhile, the authors firstly got the U-Pb age (510.60±1.40 Ma) of zircon from pyroxene peridotite and hold that its age is considered to be the Cambrian and it may represent early products of South Altun oceanic basin.