山东岚山I型与A型复合片麻状变质花岗岩: 年代学、地球化学及其构造指示意义

The I- and A-type composite gneissic metagranites with Neoproterozoic protolith ages are outcropped at Lanshan of Rizhao, Shandong Province. Mineral assemblages for the I-type gneissic metagranites are quartz + K-feldspar + plagioclase + biotite, and for the A-type rocks are quartz + K-feldspar + plagioclase + aegirine + arfvedsonite + biotite. Zircon LA-ICP-MS U-Pb dating gives protolith ages of 795 ±32Ma for I-type gneissic metagranites, and of 772 +26Ma for A-type rocks. Both the I- and A-type gneissic metagranites had been experienced ultrahigh-pressure metamorphism in Early Mesozoic and were subsequently overprinted by intensive amphibolite-facies retrograde metamorphism at about 220Ma. Geochemically, the I- and A-type gneissic metagranites are all characterized by rich in silicon and iron, and poor in magnesium, however, there are still some obvious differences between them. Compared with the I-type gneissic metagranites, the A-type rocks contain higher alkalis and lower aluminum, with alumina saturation index (ASI) values generally ranging from 0. 88 to 0. 94, thus showing a metaluminous signature. In contrast, the I-type rocks with ASI values of 0.94 - 1.01 are commonly metaluminous to weakly peraluminous. The A-type gneissic metagranites also have higher contents of REE, HFSE (e. g. , Nb, Ta, Zr, Hf, Y, Ga) , and display higher 10⁴ x Ga/Al ratios ( =2. 77 -3. 89). The two types of rocks have different Nd isotopic compositions, with ε_Nd(i) values of -3. 73 - -5. 20 for the I-type, and of -6. 26 - -8. 00 for the A-type, suggesting that they were generated from different sources. Geochemical modeling also indicates that the I- and A-type gneissic metagranites evolve along different trends. The gneissic metagranites at Lanshan show both arc-and rift-like geochemical signatures, which are quite different from those developed under an active rifting setting in association with upwelling of mantle plume. The occurrence of I- and A-type composite gneissic metagranites indicates that during the period of their protolith emplacement ( 795 -770Ma) , the northeastern margin of Yangtze Plate was under the initial stage of a passive rifting setting induced by back-arc extension.