Genesis of mafic enclaves from rapakivi-textured granites in the Qinling and its petrological significance: evidence of elements and Nd, Sr isotopes.

The enclaves from rapakivi-textured granite plutons in the Qinling orogenic belt are mainly dioritic magmatic enclaves, having low content in SiO2(50%-62%),high in K2O+Na2O(7.01%-9.4%), δ(5-9), F, REE, Cr, Ni, V and Ga. The major oxide contents of the enclaves and their host rocks plots on the same linear trends. The pattern of REE, trace element and isotopic signatures of the enclaves are similar to those of the host granites. Exchanges of active elements, HLE and LREE occurred between the enclaves and the host rocks and almost reached equilibrium. Those characteristics were resulted from the nature of magma mingling/mixing, i. e. the formation of rapakivi-textured granite plutons at least involved two kinds of magmas hybrid. Low (87Sr/86Sr)i(0.70514-0.70624), high eNd(t)(-0.95- -3.3), Cr, Ni and V of enclaves suggest that their magma of the enclave may have originated from mantle and may have been a basaltic magma in nature before its mixing with the silisic magma. The relationship between the enclaves and host rocks suggests that the way of the magma mingling/mixing is the basic magma injecting into the acidic magma. This study provides new evidence for the genesis of the rapakivi-textured granites. That is, the granites crystallized from a hybrid magma formed by mingling of the crust-derived acidic magma and the mantle- derived basic magma.