Neimenggusu zuoqi granitoids geochemistry and SHRIMP geochronology
|School||Chinese Academy of Geological Sciences|
|Course||Mineralogy,petrology, Mineral Deposit Geology|
|Keywords||Granitoids Adakite In central Inner Mongolia Chronological study of Zircon Paleo-Asian Ocean Siberian plate Granitic rocks Geochemistry Trace elements|
Granitoids are widely distributed in Sozuoqi area, Inner Mongolia. The aim of this thesis is to study the geochemical characteristics and zircon SHRIMP U-Pb ages of the granitoids.1. The geochemical characteristics of the Baiyinbaolidao tonalites, in Southern Suzuoqi, Inner Mongolia are similar to those of typical adakites, e.g., the tonalities have SiO2≥56 wt.%(61.1~61.6%), Al2O3>15 wt.%(16.6~17.1%), MgO<3 wt.%(2.3%), Na2O>3 wt.%, low Y and HREE (Y=14.5~16.2, <18μg/g; Yb=1.61~1.64μg/g, <1.9μg/g) and high Sr (604~649μg/g , >400μg/g). The tonalities are proposed to be products of young subducted oceanic crust. High-precision SHRIMP U-Pb zircon analyses indicate that the tonalites were formed at 464±8Ma ~ 479±8Ma. According to the previous SHRIMP data, the Baiyinbaolidao tonalites were possibly emplaced at 464Ma ~ 490Ma. It is suggested that the subduction event of the Middle Inner Mongolia took place in the Early Ordovician.2. The geochemical characteristics of the K-rich granites from the Suzuoqi area, Inner Mongolia are similar to those of the post-collisional high-K granites. High-precision SHRIMP U-Pb zircon analyses indicate that the high-K granites from the Suzuoqi area were formed at 423±8Ma ~ 424±10Ma, which is slightly later than the age of the adakites in Southern Suzuoqi (464Ma ~ 490Ma). It is suggested that the arc-continent collisional event of the Middle Inner Mongolia took place in the Silurian after the subduction event in the Ordovician.3. The A-type granites occurred in the Suzuoqi area, along the Suolun-Hegen suture zone are characterized by the perthitic texture and the mineral assemblage of perthite(40%±) + plagioclase(25%±) + quartz(30%±) + biotite(<5%). The geochemical characteristics of the Suzuoqi A-type granites are similar to those of typical A-type granites. For example, the granites have high SiO2 (75.88 wt.%), low CaO (0.54%), and high alkalis (K2O=4.97%, Na2O=3.69%, K2O+Na2O=8.66%); Chondrite-normalized REE patterns show significant negative Eu anomalies (δEu =0.04). The granites are typically high in Ga, Zn, Zr, Nb and Y, and low in Ba and Sr; and in the spidergrams, they show the characteristic negative anomalies in Ba, Nb, Sr, P, Eu and Ti; The average 10,000×Ga/Al ratios of the granites is 3.66, which is near to the global average of 3.75 for A-type granites. High-precision SHRIMP U-Pb zircon dating of the Suzuoqi A-type granites indicates that the A-type granites were formed at 222±4Ma. The formation of the A-type granites was related to the post-orogenic processes of the Central Asian Orogenic Belt (CAOB), probably due to lithospheric extension.4. The high-K Peraluminous granites in Suzuoqi area are characterized by positive or slightly negativeεNd(T) values and low initial 87Sr/86Sr ratios which suggest that the juvenile crustal or mantle component played an important role in their magmatic source composition. New SHRIMP zircon U-Pb ages indicate that the granites were emplaced at 222±6Ma and 204±12Ma respectively. They belong to the highly fractionated felsic I-type granites, rather than S-type granites.5. The Central Asian Orogenic Belt (CAOB) is renowned for massive generation of juvenile crust which produced by both lateral accretion and vertical accretion during the Phanerozoic. And the CAOB is characterized by very large volumes of granitic rocks, the juvenile crustal or mantle component played an important role in their magmatic source composition.