PETROLOGY, GEOCHEMISTRY AND GENESIS OF THE NA-RICH VOLCANIC ROCKS OF THE KALATAG AREA, EASTERN TIANSHAN

Abstract

Volcanic rocks of the Lower Devonian Kalatag formation in the Kalatag region, eastern Tianshan are dominated by Na-rich andesite, rhyolite and dacite rocks with less Na-rich basaltic andesite and basalt and minor basaltic andesite rocks. These rocks are characterized by high Na2 O content (mostly > 3% ) and N2 O/K2 O ratio (mostly > 2% ). A majority of the plagioclase both as phenocrysts and in the matrix of the Na-rich rocks is albite, and oligoclase is found occasionally. The phenocryst of acidic rocks consists mainly of albite and quartz, while that in mafic and intermediate rocks consist of albite with oligoclase being occasionally seen. Acidic rocks are characterized by porphyritic, microlitic and spherulitic textures, while mafic and intermediate rocks features pilotaxitic and hyalopilitic textures. All of the whole-rock CIPW calculations contain Q, Hy and Hm, but some samples contain normative Di and C. All the plots on the TAS diagram are located in the subalkline area. REE behaviour in the rocks are characterised by ∑ REE = 25.96 ~ 96.11 ppm, LaN/YbN = 1.68 ~ 5.32, Eu/Eu * = 0.46 ~ 1.25, and LaN/SmN = 1.01 ~ 2.95. The rocks are moderately enriched in most highly incompatible elements, strongly depleted in Nb and Ta, and moderately depleted, in some samples, in Cs, K, Rb, Zr and Hf. These characteristics demonstrate that these Na-rich volcanic rocks were originated by partial melting of a source in the mantle wedge, which had been reacted upon by a sodic magma resultant from partial melting of the subducting slab. Such a mechanism of petrogenesis is consistent with previous explanation that the Kalatag region is a volcanic island arc during the early Devonian.