SHRIMP Dating Of Early Paleozoic And Early Mesozoic Adakitic Plutons From Damaoqi, Inner Mongolia

Detailed zircon SHRIMP U-Pb dating, in combination with petrological and geochemical studies, has been undertaken, mainly for granitic rocks from northern Damoqi area, where these rocks were associated with early Paleozoic and early Mesozoic orogenesis. The results are described as following:1. Early Paleozoic samples (452-440 Ma) of diorite, quatz-diorite and granodiorite of the area show geochemical features characteristic of typical adakites including: (1) high SiO₂ (≥56%), Al₂O₃ (≥15%), Na₂O (>3%, Na₂O/K₂O>1), and low MgO (<3%); (2) high Sr and low Y and Yb contents, resulting high Sr/Y (mostly >40); (3) no or slightly negative Eu(δEu=0.77～0.93); (4) In the spider diagram, displaying striking positive Sr and negative Nb, Ta, Ti and P anomalies; (5) strongly enriched in LREE and depleted in HREE (La/Yb=7.19～15.80). By definition, such rocks are interpreted as partial melting products of hot, young (<25 Ma) ocean crust subducted to 70-75 km deep, where garnet is a stable residual phase during partial melting.2. One tonalite at Chaganhushao is geochemically similar to typical adakites in many aspects. However it is lower in K (K₂O<0.5 %), which requires a low-K model source, such as tholeiitic basalt. SHRIMP dating yielded an emplacement age of 417 Ma for the tonalite.3. The samples of diorite and granodiorite, which are sampled in the Chegendalai River, are geochemically enriched in LREE and depleted in HREE with positive anomaly in Sr, negative anomalies in Nb, Ta, Ti, P and inaparent negative anomaly in Eu. All these characteristics suggest that they are similar to typical adakites. They ploted in the field of calc-alkaline series area in the pattern of SiO₂-K₂O. They are proposed to be products of partial melting of the subducted MORB slab, or partial melting of the thickened basaltic lower crust. Zircon SHRIMP U-Pb dating indicates that the diorite implaced at～239 Ma and the granodiorite at 245±1 Ma, which are indistinguishable within the analytical errors. The ages are interpreted to be indicative of either asubduction event of ocean crust or an event of post-collisional extension with crustal thinning.4. The discovery of of the high K basalts nearby Xiebiehe, in Baoerhantu, provides robust evidence for post-collision. Zircon SHRIMP U-Pb dating yielded a weighted mean 206Pb/238U age of～224 Ma, suggesting that the centeral Inner Mongolia evovlved into post-collision stage at～224 Ma (Triassic). Additionally, high precise Zircon SHRIMP U-Pb dating shows that there are inherited zircons in the basalt (BEHT01, >1000 Ma, 6 grains), diorite (BYH04, 1967 Ma, 4 grains) and tonalite (>1000 Ma, 1 grain). These evidences indicate that basement or micro-continent older than 1000 Ma existed in the reaserch area.5. Zircons with weighted mean ²⁰⁶Pb/²³⁸U age of 262±5 Ma (n=4) and 260±3 Ma (n=4) were detected in the basalt (BEHT01) and diorite (BYH04) samples, showing that there is an late Permian magmatism in this area.