北淮阳中生代adakite岩石地球化学特征及成因讨论

Petrological and geochemical characteristics of Mesozoic adakite from Northern Huaiyang and discussion on its genesis.

大别山造山带后缘的北淮阳带发育大面积的中生代岩浆岩。其中,晚侏罗世至早白垩世早期(15～130Ma)的岩体具有与adakite类似的地球化学特征,SiO2>56％,高铝(SiO2>70％时,Al2O3>15％)、高锶(Sr>400μg/g)、钠质(Na2O3.34％～4.75％,Na2O/K2O>1)、低Y和Yb(Y≤18μg/g),Sr/Y>20～40,La/Yb>20,Eu无明显异常,Sr正异常。Adakite岩石类型为(石英)闪长岩、二长闪长岩-花岗闪长岩-二长花岗岩-花岗岩。矿物成分以更长石、微斜长石或正长石、镁质黑云母、石英为主,副矿物为磁铁矿+榍石十锆石+磷灰石组合,岩石含富云质和闪长质包体,属Ⅰ型花岗岩类。而早白垩世晚期<130Ma的岩体,属A型花岗岩,为非adakites花岗岩类。北淮阳adakites的形成可能与底侵到下地壳的玄武质岩石部分熔融有关,部分熔融比例在10％～>20％,并且在岩浆演化中有AFC过程。

Mesozoic magmatic rocks are widely emerged in north Huaiyang belt located in the backland of the Dabieshan orogenic belt. The geochemistry characteristics of igneous rocks formed from late Jurassic epoch to early Cretaceous epoch (150 similar to 130Ma) are similar to those of adakites with SiO2>56%, high Al content (Al(2)O(3)greater than or equal to 15% while SiO2>70%), high Sr content (Sr>400 mug/g), enrichment of Na (Na2O=3.34%similar to4. 75%, Na2O/K2O>1), low Y and Yb content (Y less than or equal to 18 mug/g, Sr/Y>20 similar to 40), La/Yb>20, Sr positive. and without obvious Eu anomaly. The adakites in this district present five rock types as (quartz) diorite, monzodiorite - granodiorite - monzogranite - granite, in which primary rock-forming minerals are aligoclase-andesine, microcline or orthoclase, Mg-biotite, quartz, and accessory minerals are assemblage of magne-titetitanite-zircon-apatite. Mica-riched and diorite-like xenoex are found in adakites, which proved that it belongs to the I-type granitoid in Origin. The igneous rocks at ages <130Ma belong to the A-type granitoid in origin instead of adakite. The origin of adakite may be ascribed to the partial melt of basaltic rocks underplated to lower crust, with the partial melt ratio at about 10->20%, and subsequently assimilation-fraction-crystallization (AFC) process occurred during the evolution of magma.