Early Paleoproterozoic Post-Collisional Basaltic Magmatism in Quanji Massif:Implications for Precambrian Plate Tectonic Regime in NW China

Basaltic magmas can provide important information about mantle source nature,tectonic settings and tectonic evolution for a given terrain.This paper reports geology,petrography and geochemistry of whole-rock major and trace elements and Nd-Sr isotopes for a suite of garnet amphibolites from southeastern Wulan(Ulan),Quanji Massif,northwestern China.The garnet amphibolites were likely generated from basaltic lavas,associated with both paragneisses and orthogneisses of the lower Delingha Group.The basaltic protolith of these amphibolites can be broadly constrained to be formed at~2.33 Ga in an extensional setting post-collision.The geochemistry of amphibolites shows subalkaline and highly evolved characteristics.They display high-Fe low-Ti characteristics,with TFeO of 13.1 wt.%-17.9 wt.%and TiO_(2) of 1.42 wt.%-3.09 wt.%(in most samples TiO_(2)≤2.5 wt.%).The chondrite-normalized REE patterns show enrichment of LREE and LILE and the primitive-mantle-normalized incompatible element patterns display negative P,Ti,Nb-Ta and Zr-Hf anomalies.The(^(87)Sr/^(86)Sr)t values of 0.6978-0.7123 andε_(Nd)(t)values of-2.81-5.08 respond to depleted mantle model ages(T_(DM))of 2.33-3.30 Ga.These suggest that the precursor magmas of the protolith of the garnet amphibolites were probably derived from the Early Paleoproterozoic depleted sub-continental lithospheric mantle that had been metasomatized by subduction-induced fluids and melts.The precursor basaltic magmas were contaminated by the older crustal components during magma ascending.This post-collisional basaltic magmatic event at~2.33 Ga in Quanji Massif thus enhanced the subduction shutdown or slowdown tectonic regime both in NW China and coevally with those plate tectonics in some important domains worldwide during the Early Paleoproterozoic.

Petrogenesis and Tectonic Implications of the Paleoproterozoic A-Type Granites in the Xiong’ershan Area along the Southern Margin of the North China Craton

Paleoproterozoic A-type granites are widely distributed in the southern margin of the North China Craton(SNCC),providing important information for understanding the Paleoproterozoic tectonic regimes in this area.This paper reports newly obtained whole-rock compositions and zircon U-Pb ages for the Tieluping syenogranite porphyry(TLP)and Huoshenmiao alkali granite porphyry(HSM)in the SNCC.Zircons from the TLP and HSM have U-Pb ages of 1805±12 and 1792±14 Ma,respectively.These ages are taken to date the emplacement of these intrusions.They had high total alkali contents(K_(2)O+Na_(2)O>7.13 wt.%),with high 10000×Ga/Al ratios(3.06–3.41)and Zr+Y+Nb+Ce values(709 ppm–910 ppm)as well as high zircon saturation temperatures(864–970℃),indicative of A-type granite affinities.High Y/Nb(1.75–3.32),Ce/Nb(7.72–9.72),and Yb/Ta(2.89–5.60)ratios suggested that TLP and HSM belonged to the A2-type granite.The negative whole rockε_(Nd)(t)values(−8.4 to−6.6)and negative zirconε_(Hf)(t)values(−15.9 to−6.3)confirmed that TLP and HSM were likely generated by the partial melting of an ancient continental crust.TheεHf(t)(−7.4 to+4.0)values of inherited zircons in the TLP suggested that they were derived from the partial melting of Archean basement rocks.Considering the geochemical similarity of the 1.80 Ga A-type granitoids in the SNCC,we propose that the TLP and HSM were formed in a post-collisional regime that was likely associated with the break-off of the Paleoproterozoic subducted slab.Upwelling of the asthenosphere provided huge heat to generate the regional 1.80 Ga A-type granite in the SNCC.

Geochemistry and Tectonic Setting of the Eshan Granites in the Southwestern Margin of the Yangtze Plate, Yunnan

The extensive Eshan granites of Yunnan are made up of three intrusive units distin- guished by their field contact relations; in descending order of age they are the Pojiao Unit, the Liizicun Unit and the Mokela Unit. The Pojiao Unit and Luzicun Unit contain petrographically and geochemically similar rocks but contact relationships show that the latter is younger. The Mokela Unit mainly consists of dykes intruding the other two and has petrographic and geochemical differ-ences. Zircon U/Pb dating and zircon crystallization temperature measurements confirm the se- quence of intrusions. Major and trace element analyses suggest that the magmas of the Pojiao Unit granites derived by partial melting of a clay-poor source from the upper crust; the magmas of the Luzicun Unit granites derived by partial melting of upper crust with a small proportion of middle crust accompanied by crystallization of albite which triggered strength reduction. Both magmas mixed and underwent with crustal contamination, assimilation and fractional crystallization. The magmas of the Mokela Unit derived from residual melts and assimilation of argillaceous rocks. A time sequence of melting, intrusion and deformation events is derived from these results and com- pared with other published tectonic models for the evolution of the SW margin of the Yangtze Plate. Magmatism was initiated by exhumation of upper continental crust during which strongly peralu- minous porphyritic biotite monzogranite granites were produced at ca. 854-852 Ma, and the genesis of two-mica granite reflected a later batch of exhumed melts with crustal contamination, assimilation and fractional crystallization at ca. 842 Ma. Finally biotite alkali-feldspar granite and tourmaline granite magmas experienced strong fractional crystallization, emplaced in the cooling stage at ca. 823 Ma, indicating the end of exhumation.

Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings:Storehouses of Rare Earth Elements

The rare earth elements(REE)are critical raw materials for much of modern technology,particularly renewable energy infrastructure and electric vehicles that are vital for the energy transition.Many of the world's largest REE deposits occur in alkaline rocks and carbonatites,which are found in intracontinental,rift-related settings,and also in syn-to post-collisional settings.Post-collisional settings host significant REE deposits,such as those of the Mianning-Dechang belt in China.This paper reviews REE mineralization in syn-to post-collisional alkaline-carbonatite complexes worldwide,in order to demonstrate some of the key physical and chemical features of these deposits.We use three examples,in Scotland,Namibia,and Turkey,to illustrate the structure of these systems.We review published geochemical data and use these to build up a broad model for the REE mineral system in post-collisional alkaline-carbonatite complexes.It is evident that immiscibility of carbonate-rich magmas and fluids plays an important part in generating mineralization in these settings,with REE,Ba and F partitioning into the carbonate-rich phase.The most significant REE mineralization in post-collisional alkaline-carbonatite complexes occurs in shallow-level,carbothermal or carbonatite intrusions,but deeper carbonatite bodies and associated alteration zones may also have REE enrichment.

Devonian alkaline magmatism in the northern North China Craton:Geochemistry,SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton（NCC）.The intrusion is mainly composed of quartz-monzonite.Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca.381.5 Ma.The rock is metaluminous with high（Na2O ＋ K2O） values ranging from 8.46 to 9.66 wt.%.The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies.The Wulanhada rocks exhibit high initial values of（87Sr/86Sr）t = 0.70762-0.70809,low εNd（t） =-12.76 to-12.15 values and negative values of εHf（t） =-23.49 to-17.02 with small variations in（176Hf/177Hf）,（0.281873-0.282049）.These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC.The Wulanhada rocks,together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions,constitute a post-collisional magmatic belt along the northern NCC.

Remelting of subducted continental lithosphere: Petrogenesis of Mesozoic magmatic rocks in the Dabie-Sulu orogenic belt

The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermedi- ate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low εNd(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon δ18O values and whole-rock δ13C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.

Post-Orogenic Granites in Pingwu Region, Northwest Sichuan: Evidence for North China Block and Yangtze Block Collision during Triassic

The Nanyili （南一里）, Laohegou （老河沟）, and Shaiziyan （筛子岩） granitic intrusions are located in the southern margin of the Bikou （碧口） block in Pingwu （平武） area, Northwest Sichuan （四川）. The petrography and geochemical characteristics of the granitic intrusions as well as their source and tectonic settings are reported and discussed in this article. The Laohegou and Shaiziyan granites are with high SiO2 （69.89 wt.%-73.05 wt.%） and Al2O3 contents, and A/CNK=1.04-1.12. They are typical strongly peraluminous granites, with supersaturation in AI and Si. The abundance of ∑REE varies in the range of （33.13-89.12）×10^-6. The rocks show an LREE enrichment pattern and obvious Eu negative anomaly. The trace element geochemistry is characterized evidently by a negative anomaly of Ta, Nb, Ti, etc. and a positive anomaly of Rb, Ba, Sr, etc.. Zircons of the Nanyili granite have higher Th/U ratios, and their CL images have internal oscillatory zoning, suggesting that the zircons of the samples are igneous in origin. The LA ICP-MS zircon U-Pb isotopic concordia diagram yields an age of 223.1±2.6 Ma （MSWD=1.4）, which indicates that the granodiorite intrusions formed in the early Late Triassic. The Nanyili, Laohegou, and Shaiziyan granites have the characteristics of post-collisional granites and are regarded as post-orogenic granites. Thus, the granite intrusions are interpreted as syn-collisional granites that resulted from the crustal thickening caused by the collisions between the North China plate and the Yangtze plate during the Indosinian. The granitic intrusions formed in a transitional environment from syn- （compressional environment） to post-collision （extensional environment）.

Zircon U-Pb Age, Geochemistry and Geological Implication of the 255 Ma Alkali-Rich Dykes from Ulungur Area, North Xinjiang

Alkali-rich dykes of the Late Permian in Ulungur （乌伦古） area are representative prod- ucts of granitic magmatism in the evolution of the Paleozoic orogenic belt in the East Junggar （准噶尔）, North Xinjiang （新疆）. We selected two representative samples for geochemical analysis （major and trace elements）, and twenty-two zircon grains for zircon dating. Isoplot （ver3.0） was used to calculate isotopic age and make concordia diagrams. This study shows that the trachy porphyry dykes, featuring low concentration of A1203, CaO, MgO and high alkali contents, are metaluminous alkaline rock and belong to A-type granitoids. The dykes have low concentration of the REE and incompatible elements, and the REE patterns show clear negative Eu anomalies （6Eu=0.74-0.58）, enriched LREE （LREE/HREE〉4, （La/Yb）N=5.97-4.63） and undifferentiated HREEs. Similar normalized REE and in- compatible element patterns are also showed in the dykes from Yemaquan （野马泉） and granites from Ulungur, suggesting that they are possibly originated from the same source and formed in the same tectonic environment, but the trachy porphyry dykes are more evolved. The age of the trachy porphyry dykes is 255.3 - 2.4 Ma, which is probably the crystallization time of the trachy porphyry. The dykes formed in late-orogenic phase of post-collision process or within-plate environment, which suggested that the trachy porphyry dykes possibly crystallized in the transition period during which the tectonic setting changed from post-collision to with- in-plate environment. So we consider that the age when the post-collision ended and the crustal cratonization begun in the East Junggar is 255 Ma, Late Permian.