The Geochemistry And Tectonic Significance Of Granitoid-dioritoid Rocks In The Western Of Langshan Mt., Inner Mongolia

The LangShan Mt. is located in the northwestern margin of the North China Block, linkage to the Yinshan-Yanshan Mt. belt at east, to Alxa block at west, to the Ordos block at south and to Siberia plate at north, and also is subject to the Central Asia Orogen. Thereby it is a very unique place which has undertaken a long-term of tectonic movements. A large amount of magma intruded in this region since Archaean, especially in Hercynian and Indo-Sinian epoch, which has played an important role on the metal mineralization. Based on the analysis for the temporal and spacial features of granitoid-dioritoid intrusions and their petrology, lithogeochemistry and isotopic chronology in the Dongshengmiao area of the western LangShan Mt., this paper discusses the magma intrusion history and its linkage to the tectonic evolution. This is critical to understanding the tectonic evolution and mineral resources exploration in the northern of North China Block. The paper obtained the following achievements:In the Dongshengmiao area of Langshan Mt., the intrusions are divided into the Late Archaean migmatitic granite and adamellite, Mesoproterozoic adamellite, Middle Carboniferous quartzdiorite, plagiogranite and a few granodiorite, Permian-Triassic adamellite and granodiorite, which are largely occurred as a stock and in NE-trending. The measurement of the single zircon U-Pb age by TIMS for 11 intrusions obtained the ages of 2485-2619 Ma,304- 321 Ma,276 Ma and 235- 246 Ma for the Late Archaean, Carboniferous, Permian and Triassic intrusions, respectively.The granitic rocks in this region are largely composed by biotite, feldspar and quartz, and also amphibolite in the dioritoid rocks. The content of alkali (Na2O+ K2O,5.5-14.9%) and Al2O3 (12.47-17.32%) in all intrusions are relatively high, they both are higher in granitoid rocks than that in the dioritoid rocks. In general, the rocks are subject to the high potassium and aluminum series. The differentiation index of the granitoid rocks are at 84.85-96.59 with the average of 92.71, and it is higher than that whose is at 49.73-78.74 with the average of 64.47 for dioritoid rocks, indicating that the granitoid rocks undertook higher grade differentiation than the dioritoid rocks. Totally, the Late Archaean thorough Middle Carboniferous intrusions are alkaline but the Permian and Triassic intrusions are calcalkaline.The granitoid rocks in this region are more richer than dioritoid rocks in Ba、K、Sr、P、Zr and LREE, but poorer in Nb, Ta, La, Nd, Hf, Sm, Tb, Lu, Yb and HREE. The ∑REE content of the granitoid rocks are higher than the diorite, and both them is richer in LREE. From Late Archaean to Late-Triassic, the ∑REE content of the intrusions increases gradually, but ratio of Eu/Sm、 La/Sm and δEu decreases in terms with enhancement of magma differentiation. This change is also found in the various rocks at the same time, i.e., the granitoid rocks are intensely differentiated than the dioritoid rocks.The analysis for the lithogeochemistry of the intrusions shows that the Late Archean to Mesoproterozoic granites were formed during the late-orgenic period, but the Middle carboniferous quartz-diorites were formed in the active plate margin, and most of Permian granites are derived from syn-collisional belt, while Triassic granites were formed during the late-orgenic period. Generally, granitoid and dioritoid rocks originated from the magmatization in the lower crust. Concerning the rock types, the Late Archaean and Permian-Triassic granitoid and dioritoid rocks can be sorted into the I-type, but the Middle Carboniferous diorite into the S-type and Mesoproterozoic granite into A-type.From Late Archean to Late Triassic, the differentiation degree of the granitoid is higher and high and the dioritoid rocks is relatively lower than granitoid rocks, particularly the Middle Carboniferous quartzdiorite is the lowest one. Since Carboniferous, the rock’s liquidus temperature suggest that the temperature when magma began to crystallize is gradually reduced from neutral to acid rock, when emplaced, the temperature of the dioritoid rocks is higher than that of graniteThe formation of granitoid and dioritoid rocks is linked to the subduction and collision of the North China Blocke with Siberia plate over the time. The magma activity reveals that the tectonic evolution in the western of Langshan Mt. is roughly divided into four phases as following: the Late Archaean continent assemblage, the Mesoproterozoic rift, the carboniferous oceanic crust subduction and the Permian-Triassic plate collision.