Ultra-high pressure metamorphism of granitic gneiss in the Yinggelisayi area, Altun Mountains, NW China

Abstract

A study on petrography and mineral chemistry of granitic gneiss from the Yinggelisayi area in the Altun Mountains of the northwestern China demonstrates that the granitic gneiss experienced a complicated multi-stage metamorphism. The peak-stage mineral assemblage is garnet+perthite(before exsolution)+titanite(before exsolution)+kyanite+zoisite +quartz/coesite ±clinopyroxene with minor apatite and rutile. The exsolution of rod-like plagioclase +amphibole is contained within the core part of coarse-grained titanite, and the Si value per unit formula of the titanite is estimated to be 1.032—1.047, which implies that the titanite contains Si with six-fold coordination and the precursor titanite is hence of supersilicic titanite and suggests that the CaSi 2 O 5 component in the titanite is 3.1%—4.7%. The P-T condition of peak metamorphism is estimated to be 3.7—4.3GPa at 1000℃. High-Al titanite inclusions are also found in the garnet. All of these suggest that the granitic gneiss underwent UHP metamorphism. The granitic gneiss has SiO 2 content (> 70%), Al 2 O 3 (12.58%—14.08%), high K2O content (>5%) with Na 2 O/K 2 O ratio of 0.4—0.6, LREE-enriched patterns with (La/Yb) N ratio of 4.3—9.1 and large negative Eu anomaly ( d Eu = 0.06—0.59), which indicate that the protolith of the granitic gneiss is the product of anatexis of the middle or upper crustal rock. The UHP granitic gneiss, together with the interbeds of UHP metamorphic garnet lherzolite and garnet-bearing intermediate-basic gneiss in the outcrops, suggests that they all might undergo continental deep subduction, which will have important bearing for further discussion of the dynamic mechanism of the formation and exhumation of the rocks in this region.