Highly fractionated I-type granites in NE china (I): geochronology and petrogenesis

摘要

Northeastern (NE) china is the easternmost part of the Central Asian Orogenic Belt (CAOB), which is celebrated for its accretionary tectonics and the world's most important juvenile crust production in the Phanerozoic era. Abundant granitoids occur in the Great Xing'an, Lesser Xing'an and Zhangguangcai Ranges in NE China. This paper presents partial results of a series of studies on the granitoids from this region,aiming to understand their role int eh building of new continental crust in eastern Asia. Three composite granite plutons (Xinhuatun, Lamashan and Yiershi) were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from granodiorite (minor), monzogranite, syenogranite to alkali-feldspar granite. Quartz and perthitic feldspar are principal phases, accompanied by minor amounts of plagioclase, biotite (<5%) and other accessory minerals. In addition, many contain abundant miarolitic cavities whichsuggest that they were emplaced at shallow levels with extensive fractional crystallization. Geochemically, the granites are silica-rich, peraluminous and have high contents of alkalis. They invariably show enrichment in light rare earch elements (LREE) and significant negative Eu anomalies. All the granitic rocks demonstrate the characteristic negative anomalites in Ba, Nb, Sr, P, Eu, and Ti, and a positive anomaly in Pb in the spidergram. The emplacement of the Xinhuatun pluton took place at 184 +- 4 Ma as revealed by zircon SHRIMP U-Pb data. This is also supported by the slightly younger Rb-Sr whole-rock (WR) isochron age of 173 +- 3 Ma. A whole-rock (WR) Rb-Sr isochron age of 154 +- 3 Ma was obtained for the Lamashan pluton, which is interpreted as close to the time of emplacement. The Yiershi pluton was intruded at about 140 Ma as evidenced by a zircon U-Pb age of 137 +- 2 Ma and WR Rb-Sr isochron age of 143 +- 5 Ma. Biotite-WR Rb-Sr isochrons and ~(40)Ar/~(39)Ar ages of feldspars allow us to estimate the cooling rate of each pluton. Geochemical data suggest that the rocks are highly fractionated I-thpe granites. Fractionation of biotite and feldspars was the principal process of magmatic differentiation and responsible for major element variation. Rb, Sr and Ba concentrations were controlled by feldspar separation, whereas REE elements were fractionated by accessory minerals, such as apatite, allanite and monazite.