Source characteristics and tectonic setting of the Early and Middle Devonian volcanic rocks in the North Junggar, Northwest China: Insights from Nd-Sr isotopes and geochemistry

摘要

Andesite and pyroxene diorite porphyry in the North Junggar, NW China yield zircon U-Pb age of41Z9 ± 1.7 Ma and 380.7 ± 3.8 Ma, suggesting that they were formed in the Early and Middle Devonian. The Early Devonian lavas consist of basalt, basaltic andesite and andesite, dacitic porphyry and rhyolite. The enrichment of LILE and LREE, remarkably negative Nb, Ta and Ti anomalies, and low (~(87)Sr/(86)Sr)_i ratios and high ε_(Nd)(t) values in these volcanic rocks, indicate that the Early and Middle Devonian igneous rocks were formed in an intra-oceanic arc, and no continental crust was involved. The wide range of lava types, abundant phenocrysts and overall chemical and isotopic coherence of the Early Devonian lavas clearly suggest that fractional crystallization was the dominant process controlling the evolution of Early Devonian lavas, and is accompanied by the reinjection of parental melt into the magma chamber which can be clearly verified by reverse zoning of plagioclase phenocrysts and their abrupt increase of An content from sieved-textured core to rim. Andesite and basaltic andesite displaying adakitic characteristics have negligible relation to melting of subducted basaltic crust; instead, they are products of basaltic replenishment and fractional crystallization of plagioclase + clinopyroxene + hornblende from mafic magmas. The mantle source of Early Devonian lavas was enriched by much more slab-derived component than that of pyroxene diorite porphyry. With the evolvement of an island arc, involvement of slab-derived components in arc magmatism could decrease; moreover, the high-level crustal contamination may be a more important mechanism than source enrichment.