Age and Nd-Sr isotopic geochemistry of the Guangtoushan alkaline granite, Hebei province, China: implications for early Mesozoic crust-mantle interaction in North China Block.

Abstract

The Guangtoushan alkaline granile occurs in the Precambrian metamorphic rocks of the North China block and contains the assemblege of quartz+alkaline feldspar+arfvedsonite+aegirine augite+aenigmatite±astrophyllite, accessory minerals are zircon, ilmenite, and chevkinite. Late pigmatitic pods in the granite are composed of coarse-grained quartz+alkaline feldspar+arfvedsonite. A whole-rock Rb-Sr isochron age is 200±16 Ma, with (~(87)Sr/~(86)Sr)_i=0.705±0.008 and MSWD=11.2, in correspondence to a cooling age. A concordia age of 220±1 Ma yielded by the single-grain zircon U-Pb dating is considered to represent the emplacement age of the granite. Geochemically, the granite is characterized by A/CNK1 and A/NK1, low Al_2O_3, MgO, CaO, Ba, Sr and high total alkali, MnO, Rb, Ga contents and pronounced negative Eu anomalies. Mineralogy and geochemistry of the granites are consistent with those of post-orogenic A-type granites. Therefore, it is suggested that the Guangtoushan alkaline granite was produced by magmatism in a post-orogenic setting. The averaged e_(Nd)(T=220 Ma) value is -8.9, higher than that for the lower crust of the North China block, implying that the parent magma for the granite could not be produced from the partial melting of the lower crust. It is also impossible that the parent magma was originated from the Precambrian high-pressure grannlite terranes in northern Hebei province, because the granulites have no evidence for partial melting. Similarly, the magma could not be derived from the present-day lower crust beneath the North China block because of its young age of 120 to 140 Ma. An enriched mantle source in subcontinental lithosphere is suggested for the Guangtoushan alkaline granite, which was contaminated by the crustal materials subducted in early Proterozoic. The early Mesozoic intrusive zone in northern North China block, including the Guaugtoushan alkaline granite, has been considered to be derived mainly from enriched mantle sources. The most intense magmatism in eastern China at around 130 Ma is in correspondence to the peak time of lithospheric thinning, but the mantle-derived magmatism before 130 Ma would be related to a process from initial to extreme lithospheric thinning. It is thus suggested that the early Mesozoic intrusions in northern North China block were formed in the early stage of lithospheric thinning. The early Mesozoic magmatism related to initial lithospheric thinning generated a post-orogenic A-type granite belt from eastern Northeast China through northern North China block to northern Alaxa region. The distribution of these early Mesozoic intrusions in either eastern Northeast China or northern North China block is limited to a long and narrow zone, indicating that not whole of eastern China was involved in initial lithospheric thinning. The lithospheric thinning would not occur widely until Jurassic and Cretaceous. It is concluded that the Meso-Cenozoic process of lithospheric thinning started temporally in early Mesozoic and spatially in northern North China block and eastern Northeast China. As more and more eastern China was involved in such a process of lithospheric thinning, the peak magmatism occurred at around 130 Ma. This would be the reason at depth for the Jurassic-Cretaceous magmatism in eastern China. In addition, the addition of mantle-derived magmas to the crust also resulted in the vertical growth of continental crust during the initial lithospheric thinning.