Adakites and Nb-enriched arc basaltic rocks (NEABs) are identified to occur within the Carboniferous arc volcanic sequence in the Alataw Mountains, Xinjiang. The adakites, which consist of calc-alkaline dacites and rhyolites, are characterized by strong depletion of heavy rare earth elements (HREEs) (e.g., Yb) and Y, high Sr contents and Sr/Y ratios, either with no Eu anomalies or obvious positive Eu anomalies, apparent positive Sr anomalies, and depleted Nb and Ti. The Alataw adakites are very geochemically similar to the adakites that were presumably derived from partial melting of subducting oceanic crust. The rhyolitic adakite in the Alataw Mountains shows low MgO contents of 0.35% and Mg# values of about 17. However, the dacitic adakite shows high MgO contents of 2.67% to 3.32% and Mg# values of 53 to 58, suggesting that the adakite was possibly contaminated by mantle peridotite. On the other hand, the NEABs are characterized by Na-rich (Na2O/K2O > 2.0), high P2O5 and TiO2 contents, positive to weakly negative Nb anomalies, and non-negative Ti anomalies, suggesting that the NEABs were probably derived from partial melting of mantle peridotite that interacted with slab melt under high geothermal gradient. The Alataw adakites were probably derived from partial melting of oceanic crust on the southern margin of the Junggar plate that was subducted beneath the Bole block in the Carboniferous. The Alataw adakites-NE- ABs association implies that the partial melting of the subducting oceanic crust and the succedent interactions between the slab melt and peridotite in the mantle wedge possibly took place under the Bole arc in Carboniferous. On the southern margin of the Junggar plate, the Carboniferous subduction of oceanic crust (basin) was possibly extensive in the late Paleozoic era. In the Alataw area, high geothermal gradient possibly occurred in Carboniferous, and partial melting of subducting oceanic crust was a probable mechanism of Carboniferous regional crust growth.
Petrology, element and isotopic geochemistry of the Mesozoic intrusive rocks in the Tongling area were systematically investigated in this study. The intrusive rocks can be divided into two groups, one contains shoshonitic rocks with SiO2 ? 55%, the other consists mainly of high-potassic calc-alkaline rocks with SiO2 > 55%. The shoshonitic rocks (SiO2 ? 55%) were generated by the fractional crystallization of the primary basaltic magma sourced from an enriched mantle, then the evolved basaltic magma likely experienced low-degree contamination with the lower crust materials when they ascended. On the other hand, although the intrusive rocks with SiO2 > 55% show most elemental geochemical characteristics similar to an adakite, such as high Na2O, AI2O3, Sr contents, high Sr/Y and La/Yb ratios, they have isotopic compositions much different from an adakite, such as low σNd(t) (-9.16—16.55) and high (87Sr/86Sr), (0.7068—0.7105), and some of them show relatively high Y and Yb contents than those of an adakite. We propose that the intrusive rocks with SiO2 > 55% were most probably produced by mixing of the mantle-derived basaltic magma and adakite-like magma derived from the melting of basaltic lower crust that was heated by the underplating mantle-derived shoshonitic magmas. The delamination of lower crust likely took place after or during the formation of these adakite-like rocks in the Tongling area.
Many of the Yanshannian intermediate-acid intrusive rocks related to Cu-Au mineraliza-tion in the Eastern Yangtze Block are characterized by high Al2O3, Sr contents, while low in Y, Yb contents, thus with high Sr/Y, and La/Yb ratios, and variational isotope signatures in particular, e.g. e Nd( t ) = -11.92—1.96, (143Nd/144Nd)i = 0.5120—0.5125, TDM = 0.70—1.71 Ga, (87Sr/86Sr)i = 0.7043—0.7076. The geochemical characteristics of these rocks suggest that: (1) these rocks are geo-chemically similar to adakite, which might have been stemmed from the partial melting of thick-ened basaltic lower crust due to basalt underplating; and (2) the high pressure (1.2—4.0 GPa) and high temperature (850—1150℃) surroundings of the lower crust favor both the fluid and ada-kite-like magma to generation. Not only can the adakite-like magma carry abundant fluid and Cu-Au ore-froming materials, but also can it bring them to the shallow part with ease and contrib-utes to the Cu-Au mineralization.
