Tectonic Magmatism Of Middle-late Paleoproterozoic In The Southwestern Ordos Block

The southwestern Ordos Block,which is the studied area of the academic dissertation,is located in Shaanxi and Gansu provinces.The geotectonic location belongs to the southwestern part of the North China Craton and the southern part of the Liupanshan tectonic belt.The southwestern Ordos Block is separated from the Qinling-Qilian orogenic belt by the Cenozoic Weihe graben in the south,and is connected with the Helan and Yinshan tectonic belts in the north.The studied area is located at the junction of stable block and active orogenic belts,and is also the key parts for understanding and studying the tectonic evolution of the Precambrian craton and the formation and evolution of the Phanerozoic orogenic belts.The research objects of the dissertation are granite porphyry,diabase and detrital zircons from the Lower Cretaceous sandstone,which distribute along the Liupanshan fault zone.The above research subjects are mainly exposed on sections of Shizuizi,Juyuan County,Ningxia Province and Tiemahe and Baijiagou,Longxian County,Shaanxi Province.Based on detailed field geology surveys,through systematic petrology,geochemistry,and geochronology studies,the spatio-temporal distribution sequence of the magmatism and metamorphism at the Middle-late Paleoproterozoic in the southwestern Ordos Block have been preliminarily established.The formation environment and geotectonic background characteristics of each rock unit in different periods are used to analyze and explore the tectonic evolution of the studied area and the entire Ordos Block during the Paleoproterozoic.The paper mainly has obtained the following results and understanding:?1?Through the field geological survey and seismic section,it is found that basement magmatic rock is located on the surface,which is similar to the shape of the veins and due to the oblique strike-slip effect of the Liupanshan fault zone.The Liupanshan fault zone controls the location of these rock masses,but does not control the formation process of them.?2?By means of the LA-ICP-MS zircon U-Pb chronological analysis,the granite porphyrys and diabases in the Tiemahe and Baijiagou sections are formed in middle and late Proterozoic,respectively.In the Temahe section,the crystallizing age of the diabase is2018±16 Ma,the crystallizing age of the rapakiwi granite is 1781±12 Ma,and the crystallizing age of the granite porphyry is 1814±12 Ma.In Baijiagou section,the crystallizing age of granite is 1792±16 Ma,and the formation age of diabase is 1804±21?3?The 2.0 Ga diabase in the Temahe section is the oldest magmatic rock discovered in the study area,and its geochemical characteristics indicate that it is formed in a mature continental rift environment.The?_Nd?t?of diabase is positive,ranging from 5.14 to 4.33,and the age of t _DM1 of Nd isotope is 2.06².15 Ga,which is consistent with its crystallization age,indicating that the magma material is directly derived from the mantle and there was a period of crustal growth events during 2.0 Ga.The geochemical and Sr-Nd isotopes of the diabase show that the magma came from the asthenosphere mantle that have not been contaminated by the crustal material or slightly altered.Through the analysis of the formation temperature and pressure conditions of the diabase,it is shown that the lithosphere and the crust of the study area was in a thinning state when the Tiemahe diabase was formed,which may suggest a mature continental rift environment.Compared with the North China Craton,the whole North China Craton has a wide range of cracking events during the period of 2.2².0 Ga.?4?The late granite and diabase formed at Paleoproterozoic?¹.8 Ga?,and the diabase can be divided into high titanium type?HT?and low titanium type?LT?.The?_Nd?t?of HT diabase is positive,and?_Nd?t?of the LT diabase and granitic rocks are negative,indicating that the magmatism is an important crustal growth event and also a transformation of the ancient crust event.Granite geochemistry is shown as A-type granites associated with rift,and two types of diabase geochemistry and Sr-Nd isotopes show that diabase magma is derived from enriched mantle that had been contaminated by crustal material.This magmatism provides the most direct evidence for the activities of the Helan aulacogen.The cracking event is in accordance with other areas in the North China Craton,and the event is related to the cracking of the Colombian supercontinent.?5?Although the 2.0 Ga and 1.8 Ga study areas are in a rift environment,according to the geochemistry and Sr-Nd isotope studies,the 2.0 Ga diabase magma source is more depleted,while the 1.8 Ga diabase source had been enriched by crust material.So the two stages of cleavage events are two phases of different geological processes.?6?After the Middle Proterozoic,the study area entered the relatively quiet tectonic activities and received sedimentation.Until the early Paleozoic,affected by Caledonian movement,the Silurian and Carboniferous deposits were missing in the study area.Late Paleozoic,affected by the Qinling orogenic belt,some magmatic rocks related to the orogenic belt developed in the southern margin of the study area,but deposition still existed in the main area.At the beginning of the Late Jurassic,the study area was rapidly uplifted,and the boulder conglomerate of the Upper Jurassic Fenfanghe Formation was deposited.A series of mantle-derived ultrapotassic magmatic rocks developed during the Late Jurassic and Early Cretaceous,which may be related to the destruction of the lithosphere of the North China Craton.Since the Late Cretaceous to the Cenozoic,the study area was mainly affected by the uplift of the Qinghai-Tibet Plateau,forming the Liupanshan fault zone and a series of brittle tectonics.