Characteristics, Petrogenesis And Minerogenic Specialization Of The Mesozoic Granitoids In Shitai Area, Anhui Province

Mesozoic magmatic intrusive rocks widely distribute in Shitai area of Anhui Province. They are mainly acidicⁱntermediate acidic granitoids. Base on the studies of petrology, petrochemistry and geochemistry of the granitoids, and comparisions with the granitoids of adjacent areas, their characteristics and petrogenesis as well as minerogenic specialization were discussed. Some preliminary conclusions have been drawn as follows:1. The spatial distribution of regional magmatic rocks is controled by the regional linear folds and faults. According to the lithofacies and their relationship, the main magmatic rock bodies, Gniujiang and Tanshan rock bodies are divided into four intrusive stages, and their lithology are respectively medium-grained quasi-prophyry (monzonite) granite→medium-grained^medium-coarse-grained moyite→medium-fine-grained quasi-prophyry granite→fine-grained or micro-grained quasi-granite-prophyry from early to late, respectively. The lithology of Dongku rock body is medium-grained or medium-coarse-grained moyite, corresponding with second stage of Guniujiang rock body. The lithology of Kushan and Wutanggang rock body is fine-grained quasi-prophyry biotite granodiorite. The parts of it have K-feldspathization.2. The magmatic intrusive rocks are high-K calc-alkaline rock serieses. For Guniujiang and Tanshan rock bodies, SiO2=72.40⁷7.33%, K₂O=4.01⁶.39%, Na₂O=2.85 ³.31%, and ALK=7.51⁹.29%, the characteristics of petrochemistry of them are similar to those of Huangshan and Fuling rock bodies in south Anhui; and for Kushan and Wutanggang rock bodies, SiO₂ =65.4⁶8.08%, K₂O=3.62⁶.99%, Na₂O=2.79³.50%, ALK=6.74⁹.78%, these characteristics are similar to those of granodiorites in Tongling area, Anhui province.3. The total REE contents of Guniujang (including Dongku) granite are very high, and those of Tanshan granite are medium. The chondrite-normalized REE patterns of them are"V"shape, enriched in LREEs, with strong negative Eu anomaly, those are similar the REE characteristics of Huangshan and Fuling granites in south Anhui; And the total REE contents of Kushan and Wutanggang are very low, The chondrite-normalized REE patterns of them are smooth curve tilted to right, enriched in LREEs, with weak negative Ce anomaly and without obvious Eu anomaly, which shows that their REE geochemical characteristics are similar to those of the granodiorite of Tongling area.4. The contents of trace element of Guniujang (including Dongku) and Tanshan granites show subtle difference but mainly similarity on the whole. They enrich in most of the lithophile elements, as well as W and Sn, the ore-forming elements. They are deplet in Cu. Their K/Rb values are low, but Rb/Sr values are high. Primitive mantle-normalized trace element patterns of them are slightly tilted to right on the whole, and show Rb enrichment and Ba, Sr and Ti depletion. These characteristics are similar to those of Huangshan and Fuling granites in south Anhui. For the Kushan and Wutanggang granodiorites, most of the lithophile elements are relatively depleted, Cu is enriched but W and Sn are not. The K/Rb values are high, but Rb/Sr values are low, Primitive mantle-normalized trace element patterns of them are tilted to right on the whole, and all of them show that Nb, Ta, Ti and Y are obvious depleted but La and Tb are enriched. The characteristics of them are similar to those of the granodiorites of Tongling area, Anhui province.5. All of these granitoids have transitional characteristics of I-type and S-type, but Guniujang (including Dongku) and Tanshan granites show mainly the characteristics of S-type granite, whereas, Kushan and Wutanggang granodiorites show the characteristics of I-type granite; They are syn-collisional and post-collisional granites which may form in compressional and the transitional from the compressional to stretch geodynamic background; The Guniujang (including Dongku) and Tanshan grianites are products of early Cretaceous magmatic activity, whereas the Kushan and Wutanggang granodiorites are products of late Jurassic^early Cretaceous period; The magmas of Guniujang (including Dongku) and Tanshan granites are products of crust partial melting, whereas, magmas of Kushan and Wutanggang granodiorites may come from mantle and mix crustal material; The partial melting temperatures of source rocks are >875℃for the Guniujang (including Dongku) and Tanshan grianites, whereas, those for the Kushan and Wutanggang granodiorites are higher relatively. The depth of magma sources of Guniujang (including Dongku) and Tanshan granites are <40km, with pressure <1.2GPa; The Guniujang (including Dongku) and Tanshan granites show ore-forming potential of W and Sn, whereas, Kushan and Wutanggang granodiorites show ore-forming potential of Au and Cu.