The geochemical characteristics of Haiyang A-type granite complex in Shandong, eastern China

摘要

Haiyang granite complex consists of K-feldspar granite and syenite, with a total exposure area of similar to 600 km(2). The K-feldspar granite is metaluminous (A/CNK = 0.70 to 0.99) and the syenite is slightly peraluminous (A/CNK 1.01 to 1.10), both of which have typical characteristics of A-type granite with high total alkali contents and FeOT/(FeOT + MgO) ratios. Zircon U-Pb age are 116.8 +/- 1.7 Ma and 115.8 +/- 2.2 Ma, for the K-feldspar granite and the syenite, respectively. This is consistent with field observation that the syenite intruded into the K-feldspar granite. Varied zircon 0 isotope (5.65-7.78%. for K-feldspar granite and 4.68-7.08 parts per thousand for syenite) with peak values that are marginally higher than those of mantle zircon reflects important mantle contributions. These together with large variation of zircon epsilon(Hf)(t) values of K-feldspar granite (-22.4 to -15.6) and syenite (-24.6 to -13.5), can best be explained by the involvement of at least two components, e.g., enriched lithospheric mantle +/- subducted materials, and upwelling asthenosphere. Apatite has right decline REE pattern. The apatite from K-feldspar granite has higher Cl contents than those of syenite, implying more influence from a subduction released fluid in K-feldspar granite source. This distinction is supported by the systematically higher oxygen fugacity of K-feldspar granite as indicated by zircon Ce4+/Ce3+ ratios. In the Yb/Ta-Y/Nb, Ce/Nb-Y/Nb diagrams, both K-feldspar granite and syenite plot in A(1)-type, with K-feldspar granite plotting closer to Ay. In the Nb-Y-3Ga and Nb-Y-Ce charts, syenite plots near the boundary between A(1) and Ay, whereas some K-feldspar granite samples plot in A(2) field, indicating a tendency of transition originally from A(2) to A(1). In general A(1) granites form in intraplate settings, whereas A(2) granite forms in post-collision. It is likely that mantle components metasomatized by subduction released fluids are easier to be partially melted, forming K-feldspar granite (closer to Ay type) with higher oxygen fugacity, which consequently eliminated subduction signatures, and then followed by A(1) type syenite. Similar to the Lower Yangtze River belt, where both A(1) and A(2) group granites of similar ages outcropped in the same region, Haiyang granite complex may also be plausibly explained by a ridge subduction model, which has been proposed as the mechanism that controlled the decratonization of the North China Craton. (C) 2014 Elsevier B.V. All rights reserved.