Geochemical constraints on the petrogenesis of the Middle Miaoershan granitoids,South China

Abstract

The Middle Miaoershan batholith in South China is one of the best examples of a composite granitic complex, comprising coarse-grained, porphyritic, biotite monzogranites, quartz monzonites, and medium- and fine-grained two-mica granites, formed respectively during the Caledonian, Hercynian, Indosinian and Yanshanian tectonic activity. This is confirmed by radiometric (Rb-Sr, U-Pb, and K-Ar) isotopic ages of 368, 296, 214, and 120 Ma, respectively. The majority of the granitic rocks are peraluminous with aluminum saturation indices [ASI, molar Al2O3/(CaO + Na2O + K2O)] of 1.05–1.48 and high 87Sr/86Sr initial ratios, i.e., (87Sr/86Sr)i (0.71012–0.72935). Chondrite-normalized (N) rare earth element (REE) abundance patterns for the granitoids are heterogeneous with (La/Yb)N between 5.75 and 14.38, variably negative Eu anomalies (Eu/Eu* = 0.24–0.80), and overall patterns similar to those of the host metasedimentary rocks. With respect to the old intrusive unit (CMG), the three young intrusive units (HXG, IDG and YG) possess generally lower ΣREE and Zr abundances, and larger negative Eu anomalies, indicating that they may be partially derived from re-melting of the CMG; plagioclase, zircon, and LREE-bearing accessory phases remained in the source during this partial re-melting of the CMG. δ18O values for all of the granitoids are relatively uniform at 10.12 to 12.99, with a mean of 11.36. The lead isotopes for K-feldspar from the granitoids have a limited variation both within and between massifs, and all of the sample points fall near the upper crust growth curve in a 207Pb/204Pb vs. 206Pb/204Pb plot, indicating derivation from supracrustal sources for rock lead and rock-forming material. The Caledonian Miaoershan granitoids (CMG), however, are proposed to have had a supracrustal metasedimentary source mixed with a minor component of infracrustal derivation, resulting in a relatively low initial (87Sr/86Sr)i value (0.70761) and ASI (<1.0). The presence of inherited zircon as ancient core relicts within young magmatic zircons in the CMG shows that partial melting of crustal material, dominantly of metasedimentary origin, combined with re-melting of the granitoids formed by previous partial melting episodes, may be the dominant processes in the genesis of the granitoids. Limited variations of their chemical compositions and Sr, Pb isotopes among different massifs are probably related to the provenance of their source materials.