CARBONIFEROUS ADAKITE-HIGH-MG ANDESITE-NB-ENRICHED BASALTIC ROCK SUITES IN THE NORTHERN TIANSHAN AREA: IMPLICATIONS FOR PHANEROZOIC CRUSTAL GROWTH IN THE CENTRAL ASIA OROGENIC BELT AND CU-AU MINERALIZATION

Abstract

Carboniferous adakite-high-Mg andesite-Nb-enriched arc basalt and basaltic andesite (NEB) suites occur in the Northern Tianshan area (Xinjiang), many are associated with copper (gold) deposits (e. g., Dabate, Axi, Tuwu-Yandong, Chihu, etc). The adakites are characterized by high Na2O/K2O ratios, high Sr but low Y and Yb contents, positive Sr anomalies, Nb and Ti depletions but no significant Eu anomalies. The high-Mg andesites (or diorites), documented for the first time in this study, are characterized by similar chemical signatures but display relatively high MgO, Cr and Ni contents. It is notable that the Axi high-Mg andesites are similar to the sanukitoid of the Cenozoic Setouchi are volcanic belt in southeastern Japan. The NEBs are sodium-rich but potassium-poor, and differ from the vast majority of normal arc basalts in their higher Nb/La ratios and minor negative to positive anomalies in Ba, Nb and Ti. The adakites have εNd(t) values (+ 3.4 - + 9.0) and (87Sr/86Sr)i ratios (0.7032 - 0.7043). The NEBs exhibit more variable εNd(t) (+ 3.6 - +11.6) and (87Sr/86Sr)i ratios (0.7007 - 0.7067). Our study suggests that the Carboniferous adakite-high-Mg andesite-NEB suites in the Northern Tianshan area are an example of the adakite metasomatic arc volcanic (or magmatic) series. The adakites were most probably derived by partial melting of subducting young oceanic crust of the Carboniferous Northern Tianshan Ocean. Additionally, slab melts and minor fluids derived from the subducting slab likely metasomatized or interacted with mantle wedge peridotites. Partial melting of the metasomatized mantle wedge peridotites was probably triggered by these processes, which led to the formation of the NEBs. Conversely, addition of a mantle component to the slab melts ultimately led to the formation of high-Mg andesites. Therefore, the Carboniferous adakite-high-Mg andesite-NEB suites in the Northern Tianshan area suggest that: (1) in the Carboniferous, the Northern Tianshan area was in an arc rather than continental rift setting; (2) lateral accretion processes must have dominated crustal growth in the Tianshan area in the Carboniferous; (3) partial melting of subducting oceanic crust played an important role in this crustal growth along with the depleted upper mantle; (4) owing to their high oxygen fugacity (∫o2), the interaction of slab-derived adakitic magmas with mantle wedge peridotites may have caused decomposition of metal sulfides, thereby allowing Cu (or Au) to enter the magmas. This process may represent the fundamental reason why Cu (or Au) mineralization is closely associated with many adakites, high-Mg andesites and NEBs in the Northern Tianshan area.