PETROGENESIS AND GEOCHEMISTRY OF CENOZOIC TAXKORGAN ALKALIC COMPLEX AND ITS GEOLOGICAL SIGNIFICANCE

Abstract

Pamir syntax is one of the areas which have the most strongly tectonic convergence and largest crustal shortening in Tibetan Plateau. Taxkorgan complex, mainly composed of Kuzigan alkaline syenites and Karibasheng subalkaline granitoids, is the largest Cenozoic alkaline complex in the Pamirs region. Aegirine-augite syenite and syenogranite constitute the Kuzigan alkaline complex. We obtain 11.1 ± 0.3Ma of Kuganzi syenites and 11.3 ± 0.6Ma of syenogranite by using SHRIMP U-Pb zircon dating. Being rich in potassium, alkali and calcium, Kuzigan complex is a series of potassic (K2O/Na2 O > 1) and alkalic granitoids. This complex is strongly rich in LREE and has high ∑ REE content, and (La/Yb)N (88-21) ratio without Eu negative anomaly. These features suggest that the magma came from the source that had no plagioclase but had garnet in its melting residue. Besides, this complex is characterized by high Ba, Sr, Sr/Y ratio, LILE and relatively low HREE, HFSE with obviously negative Nb, Ta, Ti anomalies resulted from the rutile in the residue. Based on the stability of pressure and temperature of plagioclase, garnet and rutile, and combined with the isotope characteristics, we suggest that this alkaline intrusion was the product of the partial melting of eclogite at the base of the thickened crust, which may be affected by fluids from the subducted zone. The age of the alkaline complex and the characteristics of its source region show that the crust of this region had been thickened at 11Ma, with a curst thickness over 50km.