Petrogenesis and tectonic implications of early Paleozoic granitoids in East Kunlun belt: Evidences from geochronology, geochemistry and isotopes

The East Kunlun Orogenic Belt(EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism.Here we report the petrology, bulk geochemistry, zircon Ue Pb dating and, Lue Hf and SreN d isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U-Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450 -430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their(^(87)Sr/^(86)Sr)ivalues range from 0.7059 to 0.7085, εNd(t) values from -1.6 to -6.0 and the zircon εHf(t) values show large variations from +9.1 to -8.6 with Hf model ages(T_(DM2)) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.

The giant tin polymetallic mineralization in southwest China:Integrated geochemical and isotopic constraints and implications for Cretaceous tectonomagmatic event

The Gejiu-Bozushan-Laojunshan W-Sn polymetallic metallogenic belt(GBLB)in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China.Tin polymetallic mineralization in this belt includes the Niusipo,Malage,Songshujiao,Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang-Laochang and Songshujiao granite plutons.These granites are characterized by variable A/CNK values(mostly>1.1,except for two samples with 1.09),high contents of SiO2(74.38-76.84 wt.%)and Al2 O3(12.46-14.05 wt.%)and variable CaO/Na2 O ratios(0.2-0.65)as well as high zirconδ18O values(7.74‰-9.86‰),indicative of S-type affinities.These rocks are depleted in Rb,Th,U,Ti,LREE[(La/Yb)N=1.4-20.51],Ba,Nb,Sr,and Ti and display strong negative Eu and Ba anomalies.The rocks possess high Rb/Sr and Rb/Ba ratios,relatively low initial 87Sr/86Sr ratios(0.6917-0.7101),and less radiogenicεNd(t)values(-8.0 to-9.1).The zircon grains from these rocks show negativeεHf(t)values in the range of-3.7 to-9.9 with mean TDM2(Nd)and TDM2(Hf)values of 1.57 Ga and 1.55 Ga.They show initial 207Pb/204Pb ranging from15.69 to 15.71 and 206Pb/204Pb from 18.36 to 18.70.Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios,lowerδ18O values and higherεHf(t)values than those of the zircon grains in the KafangLaochang granites.The geochemical and isotopic features indicate that the Laochang-Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke,whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source.Most zircon grains from the Songshujiao,Laochang and Kafang granites have high-U concentrations and their SIMS U-Pb ages show age scatter from 81.6 Ma to 88.6 Ma,80.7 Ma to 86.1 Ma and 82.3 Ma to 87.0 Ma,suggesting formation earlier than the monazite and cassiterite.Monazite SIMS U-Pb ages and Th-Pb ages of three same granite samples are consistent and show yielded 206 Pb/238U ages of 83.7±0.6 Ma,83.7±0.6 Ma,and 83.4±0.6 Ma,and 208Pb/232Th ages of 83.2±0.5 Ma,83.8±0.4 Ma,and 83.5±0.9 Ma,which are within the range of the SIMS zircon U-Pb ages from these rocks.The data constrain the crystallization of the granites at ca.83 Ma.In situ U-Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field,and two from the cassiterite-oxide+cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206 Pb/238U ages of 83.5±0.4 Ma(MSWD=0.6),83.5±0.4 Ma(MSWD=0.5),83.6±0.4 Ma(MSWD=0.6)and 83.2±0.7 Ma(MSWD=0.6),respectively.Combined with geological characteristics,the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval.We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous.

Chronology, Geochemistry and Geological Significance of Haizi Bimodal Intrusive Bodies in Wuding District, Central Yunnan

The Haizi diabase-granite porphyry composite rock mass is located in the Wuding-Yanjing taphrogenic trough within the Paleoproterozoic Kangdian fault-uplift zone. According to field observations, the diabase is divided into two types: central facies and marginal facies;the granite porphyry directly contacts the central facies of diabase without transitional intermediate rock. In order to disclose its geological and petrological significances, this paper carried out the LA-ICP-MS zircon U-Pb dating and geochemical investigation on both masses of Haizi diabase and granite porphyry. The results showed that the207Pb/206Pb weighted average age of granite porphyry is 1764 ± 18 Ma and diabase 1765 ± 5.4 Ma. The geochemical investigations revealed that the Haizi diabase is a kind of high-potassium alkaline basalt and the granite porphyry a kind of low-potassium rhyolite, both enriched with high field intensity elements and depleted in large-ion lithophile elements;there is obvious component intermittence between the diabase and the granite porphyry, both being bimodal and showing characteristics of continental rift valley magmatite. The?in situ?Hf isotope of dating zircon showed that mantle materials participated in the formation and emplacement of Haizi granite porphyry and crust materials participated in the formation of diabase. This meant that crustal remelting and accretion of new crust occurred in this region in the 1.7 Ga period. It can be therefore inferred that the Haizi bimodal intrusive rock came into being due to the breakup of Kunyang rift valley and rapid ascent of mantle materials in the 1.7 Ga period, which echoed the global Columbia supercontinent breakup and was the first petrological record of that breakup found at the southwest margin of Yangtze platform.

The effects of the source composition on the origin of orthopyroxenebearing adakitic granitoid in West Qinling,Central China

Orthopyroxene-bearing granodiorite(sometimes referred to as‘charnockite’)with an adakitic affinity is a rare type of granitoid.It is generally accepted that the stabilization of orthopyroxene in igneous charnockites essentially requires low aH_(2)O and/or high temperatures in a closed system.However,orthopyroxene can be an antecryst in a trans-crustal magmatic system.In this regard,orthopyroxenebearing granitoids are somewhat analogous to pseudo-charnockites,where the orthopyroxene stems from a mafic reservoir.On the other hand,the source compositions of continental adakites can vary,which is often ignored in the interpretation of their contribution to the adakitic geochemical signature.In this study,we have investigated a rare orthopyroxene-bearing felsic pluton from the Zhuyuan area of West Qinling,Central China.The Zhuyuan pluton was emplaced in the Middle–Late Triassic(222–217 Ma)and is mainly composed of metaluminous to weakly peraluminous granodiorites belonging to the high-K calc-alkaline series.Moreover,they are characterized by high Mg#values(49.7–60.9)and Sr contents(471–697 ppm),low Y(12.2–15.4 ppm)and Yb(1.03–1.24 ppm)contents,high Sr/Y(33.2–46.2)and(La/Yb)_(N)(15.3–21.4)ratios,and weakly negative Eu anomalies(Eu/Eu^(*)=0.78–0.89).The Zhuyuan adakitic granodiorites exhibit fairly limited Sr–Nd–Pb isotopic ratios and variable zircon initial Hf isotopes,indicating a major contribution from the Neoproterozoic basement of the Qinling Orogenic Belt.There is no evidence of any formation through high-pressure magmatic processes,and we propose that the adakitic signature of the Zhuyuan pluton could have been inherited from its source rocks(i.e.,from the Neoproterozoic basement).The orthopyroxenes in the Zhuyuan granodiorites display poikilitic textures with high MgO,NiO and Cr_(2)O_(3) contents,indicating that they have an antecrystic origin.Studies of regional tectonic evolution have shown that the Zhuyuan granodiorites formed during the tearing stage of the A’nimaque–Mianlue oceanic subduction slab.Therefore,this study emphasizes the effect of source inheritance on the formation of pseudo-charnockite with an adakitic signature.

Multi-stage metallogeny in the southwestern part of South China,and paleotectonic and climatic implications:A high precision geochronologic study

The South China Block(SCB)is among the large-scale W-Sn mineralized regions of the globe.The Laojunshan W-Sn-dominant ore area(LOA)in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam,and Yangtze to Cathaysia blocks,with coeval formation of giant metallic deposits.The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB.In this study,we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-schee lite mineralization.The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units.The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block(420–380 Ma)jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones.This event generated the Dulong-Song Chay granitoids,together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites(ca.419 Ma)and pegmatites(ca.389 Ma),which include the early-stage Sn-sulfide skarn(ca.418 Ma)and the late-stage Sn-bearing schist(ca.389 Ma).During the Late Permian to Late Triassic(260–220 Ma),with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east,the SCB and Indochina Block(IB)were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit.The subducted PaleoTethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume(270–259 Ma),which formed the Maguan diabase(ca.260 Ma)that shows significantly older Re-Os model age of ca.268 Ma,suggesting that the Nanwenhe mineralization is potentially derived from ca.260 Ma source.Furthermore,the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism(ca.230 Ma),with the main stage of scheelite-Sn-Mo mineralization(229.9,229.8 and 219 Ma)and contemporary formation of the pegmatite(230.7 Ma).The Late Cretaceous involved two episodes of alternate extension and shortening,driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the NeoTethys seafloor.The evolution of the LOA consists of the NW–SE transpression ending ca.100 Ma,the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma,the WNW–ESE transpression beginning at ca.85 Ma and the NAS extension in the later episode during the latest Cretaceous,which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization,with ages in the range of 90–89 Ma,87–85 Ma and 83–79 Ma.We also evaluate the implications of magmaticmetamorphic-metallogenic degassing on the regional paleoclimatic history.

Petrogenesis of granitic pegmatite veins:Perspectives from major element and B isotope in tourmalines,Chakabeishan,Northern Tibetan Plateau

The petrogenesis of regionally zoned granitic pegmatite veins remains debated.Because of the economic significance,we carried out a study on the Chakabeishan(CKBS)pegmatite-type Li-Be deposit,eastern North Qaidam Tectonic Belt,Northern Tibetan Plateau,by means of in-situ major element and B isotope compositions of tourmalines in the beryl-bearing and spodumene-bearing pegmatite veins.Tourmalines(Tur-Be)from the beryl-bearing pegmatite are homogeneous schorl with low Mg/(Mg+Fe),high Na/(Na+Ca)and Y Al,suggesting that they are of magmatic origin.Two generations of tourmalines(TurLi)from the spodumene-bearing pegmatite are identified:(i)the crystal cores(mostly elbaite and Lirich schorl with subordinate schorl)are consistent with being of magmatic origin crystallized at the magmatic stage;(ii)the crystal rims(schorl)are best understood as the overgrowth at the later hydrothermal stage.Tur-Be and Tur-Li show an obvious difference in core-to-rim B isotopic variation trend with d11B decrease in Tur-Be and increase in Tur-Li.The core-to-rim d11B decrease in Tur-Be results from degassing during its host pegmatitic melt evolution,whereas the core-to-rim d11B increase in Tur-Li is related to fluid exsolution.The estimated d11B values for the initial melts of the beryl-bearing and spodumenebearing pegmatites are10.46‰and10.78‰,respectively,indicating that they most likely originate from protracted fractional crystallization/differentiation of granitic intrusions rather than partial melting of metapelite.Both Mg/(Mg+Fe)ratios and Li abundances in the cores of Tur-Be are lower than those of Tur-Li,suggesting that Tur-Li crystallizes from chemically more evolved melts.