Petrogenesis And Tectonic Significance Of Early Paleoproterozoic Granitoids In The Quanji Massif

The2.4-2.2Ga collision-relate magmatism is generally considered to be weak or lacking in the evolutionary history of the earth, interpreted to represent the stagnation and deceleration of plate subduction. However, large-scale intermediate to acid intrusive rocks at2.4-2.2Ga are widely reported in the Trans-North China Orogen (TNCO), Tarim Craton (TC) and Quanji Massif (QM) in China, in addition to those in Canadian Rae Cratonand Churchill, Brazilian Sao Francisco Craton and Amazonian Craton and Congo Craton in West Afica. Therefore magmatism in this period is widespreading in a global scale with significant Geodynamic information, and the related tectonic regimes can provide important clues for understanding evolution of the paleo-continents in the Paleoproterozoic.The QM, an important member of the micro-continents to the southeast TC in northwest China, is located to the jointing position for the TC, North China Craton (NCC) and Yangtze Craton (YC). It is interpreted as an ancient cratonic remnant detached from the TC, composed of metamorphosed pre-Neoproterozoic basement and overlying Nanhua-Sinian strata. The～2.4Ga granitic gneisses occur in the Delingha Complex of the lower basement and mainly exposed in the middle and east QM. The～2.2Ga volcanic rocks are observed in the middle part of the Dakendaban upper subgroup in middle QM. Although study was carried out for the granitic gneisses in2002, it was confined in regional geological survey. The resultant data are lacking or limited, without comprehensive analysis and interpretation, or simply discussing with diagrams. Thus available conclusions are conflict with regional tectonic history. In this thesis, based on previous data, a comprehensive study is taken on geology, petrology, geochronology, geochemistry, whole-rock Sr-Nd and zircon Lu-Hf isotopes of the Paleoproterozoic granitic gneisses in the middle and eastern QM to understand their distributions, rock types, intrusive ages, petrogenesis, magma sources and tectonic settings, in order to provide further insights to the crustal growth and tectonic affinity of the QM in the early history, and the Paleoproterozoic tectonic relations of the QM to the TC and NCC and even the global paleo-continents.The metamorphosed Paleoproterozoic granitic gneisses in middle and eastern QM comprises three plutons including Delingha, Hudesheng-Liudaoban and Mohe granitic gneisses; they are located in the north and east Delingha City, the Hudesheng Hill to Liudaoban area and the Narengou to the east Wulan County, respectively. These plutons are all intensively deformed and metamorphosed, but their intrusion relationships are still clearly recognized, with palimpsest textures such as blastoporphyritic texture, blasto-plagioclase-inclusion texture and relict magmatic minerals like allanite, thus are appropriately described as plutons. Blastoporphyritic textures are distinctly observed in both outcrops and thin sections, indicating that the protoliths were granites. The Delingha and Hudesheng-Liudaoban intrusios occur as batholiths (>150km2) and stock (60-70km2), respectively. They contain similar rock types, including mainly the syenogranitic, monzogranitic, adamellitic and granodioritic gneisses. The Mohe intrusion appears is apophyse (～10km2), with metamorphosed rocks dominantly of tonalite-diorite and granodioritic gneisses and minor adamellitic gneiss. Coarse-grained subhedral-euhedral magmatic biotite, platy plagioclase and K-feldspar, concentric zoned allanite are well preserved in the Delingha intrusion. In the Hudesheng-Liudaoban intrusion, metamorphic minerals including garnet, hornblende and biotite developed due to involving in the Early Paleozoic North Qaidam Subduction Complex Zone. metamorphic minerals like epidote, zoisite, fine-grained breen biotite, muscovite and chlorite grew in the retrograde stage, constituting the superimposing gneissic schistosity. Magmatic plagioclase in the Mohe gneiss can be divided into three generations:the first one is the lathy crystals included in the hornblende, the second one occurred as blasto-sphenocrysts setting in the matrix of quartz and the third generation plagioclase. The peak conditions of the Paleoproterozoic metamorphism are upper-amphibolite facies, consistent to those of the crystallization conditions, thus didn’t change magmatic mineral association. In retrograde stage, epidote-amphibolite facies minerals including blue-green hornblende, green biotite, epidote, zoisite, chlorite and albite were generated.CL image-Based zircon U-Pb isotope determination by LA-ICP-MS yielded2367±12Ma and2398±4.3Ma for zircons from two samples from the Delingha intrusion,2395±48Ma,2382±37Ma,2389±26Ma and2368±12Ma for zircons from four samples from the Hudesheng-Liudaoban intrusion, respectively. The determination also yielded2394±31Ma and2392±25Ma for zircons from two samples from the Mohe intrusion, which is considered to be better than previous age of2341±33Ma, and confirms that the previous data of2470Ma represents the crystalline age of the zircons from an early magmatic rock enclave. These data constrain the granitic gneisses including the Delingha, Hudeshend-Liudaoban and Mohe intrusions in the QM crystallined at～2.37-2.39Ga, and at the same time confirm that the previous age of2202Ma for zircons from the Hudesheng-Liudaoban intrusion is not representative of the emplacement age but a meaningless one affected by metamorphic recrystallization. However, volcanic activities at～2.2Ga did exist, certified by magmatic inherited detrital zircons from the middle unit of the upper Dakendaban subgroup.The Delingha, Hudesheng-Liudaoban and Mohe intrusive rocks were post-collisional granitoids formed in the post-collisional transitional stage from compression to extension. The Delingha granitic rocks has relatively high Si, K, and alkali-rich but low Ca contents, and high Fe/Mg and Ga/Al ratios with A/CNK=0.88-1.09. It is rich in LILE (e.g. Rb, Th and K) and HFSE (e.g. Zr, Y and Ga), but Ba, Sr, Eu and Nb depletion, REE patterns show a typical right-wing plan"seagull-type". The Hudesheng-Liudaoban granitic rocks also have high Si, K. and alkali and higher Al and Ca contents but none Ba depletion with high Fe/Mg and Ga/Al ratios and A/CNK=0.91-1.00. It is rich in HFSE (e.g. Nb, Zr, Y and Ga) but strongly depleted in Sr, P and Ti with weakly Eu anomalies, showing a typical right-wing plan"seagull-type" REE pattern. These two granitic rocks are not associated with alkali granite, syenite and quartz-syenite, meanwhile contain magmatic biotite (not annite) and hornblende/muscovite, indicating that they belong to I-type granites with some geochemical characteristics of A-type granites, not members of bimodal suites. Therefore, the magmas of these two granitic rocks might be formed within an extensional tectonic regime in the late-or post-collision setting. The Mohe granitic gneisses have relatively high Si, Al, K and Sr contents and Sr/Y and La/Yb ratios but low MgO, Cr, Ni and Y contents and Mg#values with A/CNK=0.79-0.93. It is rich in LREE but depleted in HFSE, with slightly or none negative Eu anomalies. In general, it was belong to adakite or C-type adakitic rock of I-type granites, of which the magma was probably produced in an extension regime related to post collisional crust thickening.The Delingha granitic rocks haved(t) values of-0.7to+4.8, with some higher values approaching to the contemporaneous depleted-mantle value (+4.9). The single-stage Nd depleted-mantle model ages (TDM1) range from2.37to2.78Ga, and the two-stage model ages (TDM2) is2.37-2.81Ga. The Zircon εHf(1) values vary from-1.0to+3.8, which correspond to the Hf depleted-mantle model ages(TDMI) between2.55and2.74Ga, and Hf crustal model ages(ToM2) of2.69to2.99Ga, with two common main age peaks of-2.78-2.82Ga. Nd-Hf isotope characteristics of the Delingha granitic rocks imply that the source magmas were mostly generated by the Neo-Archean (-2.7-2.8Ga) juvenile crust and a minor-2.4Ga juvenile crust material.The Hudesheng-Liudaoban granitic rocks have slightly low (87Sr/86Sr)i=0.703-0.706, and ENd(t) values from+0.4to+2.6, TDMI ranging from2.55to2.73Ga, and TDM2from2.57to2.73Ga. Zircon εHf(t) values vary from+0.1to+7.8, with peak values at+2.0to+5.0. A few zircons have values of εHf(t) close to the contemporaneous depleted mantle evolutionary line (+4.9). The Hf-isotope delepted-mantle model ages (TDMI) range from2.42to2.71Ga and the crustal model ages TDM2of2.44to2.85Ga, with main age peaks of2.57-2.80Ga, which is consistent with the whole rock Nd model ages. Sr-Nd-Hf isotopes of the Hudesheng-Liudaoban granitic rocks indicate that the late Neo-Archean(-2.45-2.8Ga) juvenile crust material, old crust and ancient crust were an important source for the magmatism.The Sr-Nd isotopes of the Mohe granitic rocks are as follows:(87Sr/86Sr)i=0.705-0.709, εNd(t)=+2.4-+4.4,TDM1=2.42-2.56Ga,TDM2=2.43-2.59Ga. The higher εNd(t) values approaching to the contemporaneous depleted-mantle value (+4.9).. Zircon εHf(t) values are in the range of+0.4to+7.6, and the highest value(+7.6) approaching to the contemporaneous depleted mantle value. The Hf isotope depleted mantle model ages (TDM1) range from2.43to2.70Ga and the crustal model ages (TDM2) range from2.46to2.92Ga, with main peak age at～2.64-2.82Ga. Sr-Nd-Hf isotopes of the Mohe granitic rocks imply that the source magmas were generated from the late Neo-Archean (-2.5-2.8Ga) juvenile crust material and old crust.Both zircon Lu-Hf and whole-rock Nd isotope characteristics of these three granitic rocks indicate a main Neo-Archean (～2.5-2.8Ga) crust growth event which mantle-derived magma input into the crust in the QM. The weakly intermediate (Mohe granite) to acid (Delingha and Hudesheng-Liudaoban granites) magmas might be formed by partial melting of these juvenile crust and old crust at-2.40Ga, and emplaced into middle-upper crust, resulting in increasing the volume of middle-upper crust according to the mafic juvenile crust turned into felsic granites. In addition, A few higher εHf(t) and εNd(t) values are even close to the contemporaneous depleted mantle values with correspond to TDM2ages at～2.4Ga and-2.45Ga, which are consistent with the emplacement age of the granitic magma at～2.4Ga, indicating the inputting of juvenile crust materials accompany with granitic magma upwelling.In combination with early Precambrian magmatic-metamorphic and crust growth events in the QM, TC, and the NCC, especially the TNCO, the QM might share very similar or close affinity to TC and NCC during the Paleoproterozoic, and they all have2.2-1.9Ga khondalite series, experienced～2.37-2.39Ga granitic magmatic events,～2.2Ga magmatic events, and main-2.5-2.8Ga and-2.2Ga crust crust growth events, as well as two metamorphic events at1.96-1.80Ga. Moreover, the intrusion age (1.7-1.8Ga) of the Yingfeng rapakivi granite in the western QM is consistent with the Miyun rapakivi granite in the NCC, suggesting a similar continental break-up history.As a while, the QM(2.37-2.39Ga), the TC and NCC were possibly in a post-collisional setting with a transitional stage from compression to extension regime, but not a continental break-up setting at2.4-2.2Ga. Before～2.4Ga, a significant subduction-collision-assembly event possibly occurred in the NCC, TC and QM and turned into a united stable continent.