Petrology and Geochemistry of the Huangshan Granitic Intrusion in Anhui Province,Southeast China:Implications for Petrogenesis and Geodynamics

The Huangshan granitic intrusion in Anhui province,SE China,is tectonically located at the southeastern boundary of the Yangtze Block.Based on the contact relation and the petrography,the Huangshan granitic intrusion can be divided into four stages,from early to late,medium-grained monzogranite,coarse-grained porphyric granite,fine-medium grained porphyric granite,and finecoarse grained granite.All rocks from the Huangshan granitic intrusion display similar petrological and geochemical characteristics,i.e.relatively high SiO_2（〉75%） and alkali（7.85%-8.59%）,low CaO （〈1%）,high Fe-number（FeO_T/（MgO＋FeO_T） = 0.93-0.97） and A/CNK（atomic Al/（Ca＋Na＋K））=1.04- 1.19.They are also enriched in rare earth elements（REE,except for Eu,with a total REE contents ranging from 116 ppm to 421 ppm）,high strength field elements such as Zr,Hf,Nb,but depleted in Ba,Sr and Ni.The 10 000×Ga/Al ratios are higher than 2.6,which are consistent with the A-type granitoids.Based on the classification diagrams proposed by Eby,the Huangshan granite can be classified into the A2 group,which is usually believed to be formed under an extensional tectonic setting.Their Nd isotopic compositions suggest that the primary magmas of the Huangshan granite are predominantly derived from the Proterozoic andesitic rocks in the region,and this conclusion is also supported by REE modeling.The systemic investigations on the geochemistry of the Huangshan granitic intrusion can provide significant implications for the understanding of the petrogenesis and the geodynamic regime of southeastern China during the Late-Mesozoic.

Extensional Setting of Hainan Island in Mesoproterozoic:Evidence from Granitic Intrusions in the Baoban Group

The Mesoproterozoic Baoban Group is the oldest basement in Hainan Island and has played an important role in Columbia(Nuna)supercontinent reconstructions.The Mesoproterozoic granitic intrusions in the Baoban Group are the most widely-exposed Precambrian magmatic rocks and are the key to understanding the tectonic settings of Hainan Island and its relationship with the South China Block and the Columbia supercontinent.New LA-ICP-MS zircon U-Pb dating on three mylonitic granite samples from the Tuwaishan and Baoban areas yield ages ranging from 1447 Ma to 1437 Ma,representing the absolute timing of the emplacement of the granitic intrusions.Combined with previously published geochronological data for rocks from the Baoban Group and regional mafic intrusions,it is concluded that the Baoban Group formed at 1460-1430 Ma,coeval with the emplacement of the granitic and mafic intrusions.New in-situ zircon LuHf isotope analyses for the three mylonitic granite samples yielded positiveε(t)values,ranging from+0.49 to+8.27,with model ages(T~C)ranging from 2181 Ma to 1687 Ma,suggesting that the granitic intrusions originated from a mixed source of Paleoproterozoic crust with juvenile crust.New zircon trace element data show characteristics of high Th/U values of 0.24-1.50,steep slopes from LREE to HREE and negative Pr,Eu anomalies with positive Ce,Sm anomalies,representing typical magmatic zircons formed in continental crust.Compared with available magmatic and detrital zircon ages from Precambrian rocks in the Cathaysia Block,Yangtze Block and western Laurentia,it is inferred that Hainan Island was separated from both the Cathaysia Block and the Yangtze Block,instead being connected with western Laurentia in the Columbia supercontinent.Considering the decreasing tendency of basin deposition time along the western margin of Laurentia,it is proposed that Hainan Island was located to the north or northwest of the Belt-Purcell Supergroup,along the western margin of Laurentia,during the breakup of the Columbia supercontinent.

Geomorphosites in Granitic Environments： An Example from Northern Sardinia （Italy）

A rare geomorphosite and a pillow-like structure appearing as mingling structures due to that mafic and acidic magma interactions are described. Despite the occurrence of spectacular forms in the granitic basement of Sardinia, the morphosites of geological interest linked to the complex evolution of pluton emplacement are still poorly understood. The Fizza Ona outcrop, located near the northeastern coast, is a good example of a geomorphosite in the inner zone developed at the edge of different plutonic units. This outcrop is a well-exposed, pillow-like horizon of less than 5 m in thickness, observable in the field for at least 200 m and underlined by the occurrence of a large tafoni form. In detail, the pillow-like horizon consists of sub-roundly to sub-rounded elements of granitic to micropegmatitic rocks of up to 100 cm in diameter, which is dispersed in the fine-grained granitic host-rock matrix. The outcrop represents a particular example of the emplacement mechanism of granitic magmas in the Sardinian batholith. One aim of this work is the construction of a document for the competent authorities to protection and preservation of this outcrop as a cultural heritage for the history of the geological evolution.