Carboniferous ridge subduction in the Xingmeng Orogenic Belt:Constraints from geochronological,geochemical,and Sr-Nd-Hf isotopic analysis of strongly peraluminous granites and gabbro-diorites in the Xilinhot micro-continent

The Xingmeng Orogenic Belt evolved through a long-lived orogeny involving multiple episodes of subduction and accretion.However,there is a debate on its tectonic evolution during the Late Paleozoic.Here,we report geochemical,geochronological,and isotopic data from strongly peraluminous granites and gabbro-diorites from the Sunidzuoqi-Xilinhot region.Zircon U-Pb ages suggest that the intrusive rocks were emplaced during the Early Carboniferous(333-322 Ma).The granites exhibit geochemical characteristics similar to S-type granites,with high SiO_(2)(72.34-76.53 wt.%),Al_(2)O_(3)(12.45-14.65 wt.%),and A/CNK(1.07-1.16),but depleted Sr,Nb,and Ta contents.They exhibit positiveε_(Nd)(t)andε_(Hf)(t)values(-0.3 to 2.8 and 2.7-5.7,respectively)and young Nd and Hf model ages(T_(DM2)(Nd)=853-1110 Ma and T_(DM2)(Hf)=975-1184 Ma),suggesting that they may be the partial melting products of heterogeneous sources with variable proportions of pelite,psammite,and metabasaltic rocks.The meta-gabbro-diorites from the Maihantaolegai pluton have low SiO_(2)(47.06-53.49 wt.%)and K_2O(0.04-0.99 wt.%)contents,and demonstrate slight light rare earth element(REE)depletion in the chondritenormalized REE diagrams.They have high zirconε_(Hf)(t)values(14.41-17.34)and young Hf model ages(T_(DM2)(Hf)=230-418 Ma),indicating a more depleted mantle source.The variations of the Sm/Yb and La/Sm ratios can thus be used to assess the melting degree of the mantle source from 5%to 20%,suggesting a quite shallow mantle melting zone.We propose that the petrogenesis and distribution of the strongly peraluminous granites and gabbro-diorites,as well as the tectonic architecture of the region,can be explained by a ridge subduction model.Based on these results,and previous studies,we suggest a southward ridge subduction model for the Sunidzuoqi-Xilinhot region.

Petrogenesis of the Early Jurassic Longtang and Menglong Peraluminous Granites in Tengchong Terrane,and their Tectonic Implication

A comprehensive study of zircon U-Pb geochronology,in situ Hf isotopes,whole-rock major and trace element geochemistry,and Nd isotopes was carried out for two Early Jurassic two-mica granites(Longtang and Menglong)in the southern part of the Tengchong terrane,which is in the northern part of the larger Sibumasu terrane.We assess the origin of the granites and explore their possible genetic relationship to the Paleo-Tethyan regime.LA-ICP-MS zircon U-Pb dating shows that they were simultaneously emplaced in the Early Jurassic(ca.199 Ma).They have SiO2 contents of 69.7–75.1 wt%and are mainly strongly peraluminous with alumina saturation index(ASI)values ranging from 1.06 to 1.46.They show similar Mg#(0.29–0.42)to experimental partial melts of metasedimentary rocks under continental pressuretemperature(P-T)conditions.They are enriched in light rare earth elements(LREEs)relative to heavy rare earth elements(HREEs),with moderately negative Eu anomalies and flat HREEs patterns.They show negativeεNd(t)values(−9.0 to–12.4)andεHf(t)values(−8.0 to−9.1).Elemental and isotopic data suggest that they most likely to formed by muscovitedehydration melting of a metapelitic source at lower temperatures in the range of 700℃to 750℃.The granites might represent a post-collisional tectonic setting response to Paleo-Tethyan regime.

Geochemistry of Micas from Issia Granite Complexe: A Marker of Geodynamic Evolution

The granites and pegmatites located in the southern part of the Issia region, near the columbo-tantaliferous placers, are characterized by the presence of rare metals such as beryl, lithium and Nb-Ta oxides. They mainly consist of micas, quartz, plagioclase and potassium feldspar. The work carried out on the micas of these granites and pegmatites (EPMA analyses) has provided new geochemical data contributing to the understanding of the magmatic evolution of the Issia granite complex. Mineralogically, the most evolved G3 granites are characterized by their abundance of muscovite compared to biotite and the presence of pegmatite veins. Geochemically, the muscovites of the G1 and G2 granites are more ferriferous than those of the G3 granites, however, the latter display higher Na contents than the G2 and G3. The muscovites of the granites show an evolution from the pure muscovite series to the zinnwaldite series (micas of the pegmatites) which are lithium-bearing micas. The mineralogical and chemical data of the micas show that they are S-type peraluminous granites and demonstrate the formation of granites and pegmatites through fractional crystallization of the same parental magma.

Early Ordovician strongly peraluminous granite in the middle section of the Yarlung Zangbo junction zone and its geological significance

LA-ICP-MS zircon U-Pb isotopic dating and rock geochemical analysis were done of the Xarru granite in the middle section of the Yarlung Zangbo junction zone. Zircon 2-6pb/Z38u weighted mean ages of 474.9±2.3 and 478.3±1.7 Ma have been obtained for two gneiss granite samples respectively, which represent the formation age of the granite. This is the first discovery of the Early Ordovician magmatism in the Yarlung Zangbo junction zone. The rocks are high-K calcic-alkalic granite, contain tour- maline but not hornblende, with aluminum saturation index （ASI） of A/CNK〉I.1 （1.10-1.20）, and are enriched in Rb, Th and U and relatively depleted in Ba, Nb, Sr, Zr, Ti and Eu. They are strongly peraluminous S-type granite, resulting from partial melting of argillaceous components in the crust in a syn-collisional setting. According to previous studies as well as the analy- sis in this paper, the formation of the Xarru granite is probably related to the Andean-type orogeny in the process of subduction of the Proto-Tethys Ocean towards the Gondwanaland, and it is a product of partial melting of the thickened upper crust as a result of collision between blocks or micro-blocks in the northern margin of the Gondwana supercontinent in the process of oceanic subduction. The Xarru granite is identified as the Early Ordovician granite, indicating that the wall rocks had probably formed in the Cambrian or Precambrian.

Zircon SHRIMP U-Pb geochronology,geochemistry and petrogenesis of the upper Eocene Shuangmaidi peraluminous granite in Baoshan Block,Western Yunnan Terrain,southwestern China

The Baoshan Block is tectonically located in the middle segment of the Sibumasu plate. Granitic magmatism within the Baoshan Block has been considered weakly active due mainly to very limited exposures during the Himalaya orogenic episode. The geochronological study on the buried Shuangmaidi granite has confirmed the existence of the Cenozoic granitoids in the Baoshan Block. The present study indicates that： （1） It is medium- to coarse-grained two mica phyric granite, characterized by high SiO2 （73.55%-77.16%） and low CaO （0.34%-1.38%） contents, with a total alkalis （K2O＋Na2O） of 5.22%-8.03%, K2O/Na2O ratios of 0.24-1.79, and total rare earth elements （ZREE） of the granite between 85 and 125 ppb. All samples are enriched in light REE and exhibit medium negative Eu anomalies; and they show pronounced negative anomalies in Ba, Sr, Ti, and Nb but significant positive anomalies in K, Rb, U, Th, and Pb on mantle-normalized trace element patterns, indicating typi-cally peraluminous to strongly peraluminous S-type granite. （2） The zircon SHRIMP U-Pb ages of the granite are 36.27±0.48 Ma for the samples from ZK7-1 and 35.78±0.49 Ma for those from ZK0-1, respectively. The similar zircon ages from these two drill cores may suggest that the granite samples come from the same buried pluton. （3） 206pb/204pb values of the granite vary from 20.115 to 25.359, 207pb/204pb from 15.776 to 16.160, and 208pb/204pb from 39.236 to 41.285, showing the characteristics of radio- active lead anomaly of the upper crust. The （87Sr/86Sr）i values calculated on the average age of the two-mica orthoclase granite （36 Ma） range from 0.72524 to 0.77503 and eNd（t） values vary from -10.9 to -11.7. These data, along with the depleted-mantle Nd modal ages of 1.73-1.80 Ga, imply that the granites might have formed from partial melting of the Precambrian crystal basements. （4） On the Hf-Rb-Ta diagram, almost all the samples fall within the field of post-collision tectonic setting. The CaO/Na2O and A1203/TiO2 ratios suggest that the granitic magma may have formed from partial melting of clay-rich crustal materials with a pos- sible melting temperature of about 900℃ and a possible crystallization temperature of 775-795~C. （5） During the post-collision of the Himalaya orogen, with the southeastward extrusion of the Indochina continent resulting from the continuous northward indentation of the India continent into the Asia, the Gaoligong Fault, as the western boundary of the Indochina continent, moved in the dextral strike-slip on a large scale to trigger partial melting of the thickened crust, and the peraluminous granitic magma from which the Shuangmaidi two-mica orthoclase granite derived was formed.

Early Cretaceous A-type Granites in the Dandong Area,NE China and its Geodynamical Implications

A-type granites,as an important petrologic sign to build up environmental recognition,were mainly formed in extensional tectonic settings.A biotite syenogranite from the Fenghuangshan pluton in Dandong,Liaoning Province gave SHRIMP zircon U-Pb ages of 122.5±1.6 Ma,124.9±1.7 Ma and 126.9±1.1 Ma.The monzogranites have SHRIMP zircon U-Pb ages of 118.2±1.6 Ma,128.1±1.7 Ma and 131.6±1.9 Ma,giving an emplacement age for the pluton in the Early Cretaceous.SiO_(2) is 65.48-74.49 wt%,whereas that of K_(2)O is 4.16-6.44 wt%,and that of Na_(2)O is 2.99-4.70 wt%.They also contain 13.24-15.76 wt% of Al_(2)O_(3),with an A/CNK ratio of 0.92-1.10,averaging 1.02.The alkalinity rate(AR)ranges from 2.68 to 5.12,and this range is within the AR of peraluminous type rocks.The granites of Fenghuangshan pluton are characterized by high contents of Na and K and low contents of Ca(thermophile element)and Mg,which are features of A-type granites.The(Na_(2)O+K_(2)O)-Fe_(2)O_(3)^(*)×5-(CaO+MgO)×5 discrimination diagram also shows that Fenghuangshan pluton is an A2-type granite.The above granite has zircon ε_(Hf)(t)values ranging from-17.06 to-9.09,with single-stage Hf model ages(T_(DM))of 1141 Ma to 1498 Ma and two-stage Hf crustal model ages(T_(DM)^(C))of 1762 Ma to 2263Ma.A comprehensive analysis suggests that the Fenghuangshan pluton might have been formed from the lithospheric-plate sliding during the late stages of evolution of hot rift structures and might have been closely associated with the tectonic settings of Mesozoic Eurasia and the ancient Pacific Plate.

Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting

The giant Central Asian Orogenic Belt is an extensive accretionary orogen,of which the Solonker suture,as a major regional suture,coincides closely with an early Permian paleobiogeographical boundary.This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane.Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian-Early Triassic,uncertainty remains because of a lack of typical collisionrelated features(e.g.,high-grade regional metamorphism and well-developed fold-thrust structures)and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes(288-275 Ma)and Middle-Late Permian undeformed layered gabbros,strongly peraluminous granites,and I-type granites(265-254 Ma)in the Xinhure area along the northern margin of the North China Craton.The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton.In contrast,the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle-upper crust,respectively,triggered by asthenospheric upwelling.Therefore,a transition from an end-compressional to an extensional environment according to a transition from collision termination to postcollision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma.This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.

Diversity of Mesozoic tin-bearing granites in the Nanling and adjacent regions,South China:Distinctive mineralogical patterns

The Nanling and adjacent regions of South China host a series of tin deposits related to Mesozoic granites with diverse petrological characteristics. The rocks are amphibole-bearing biotite granites, or （topaz-） albite-lepidolite （zinnwaldite） granites, and geochemically correspond to mealuminous and peraluminous types, respectively. Mineralogical studies demonstrate highly distinctive and critical patterns for each type of granites. In mealuminous tin granites amphibole, biotite and perthite are the typical rock-forming mineral association; titanite and magnetite are typical accessory minerals, indicating highjO2 magmatic conditions; cassiterite, biotite and titanite are the principal Sn-bearing minerals; and pure cassiterite has low trace-element contents. However, in peraluminous tin granites zirmwaldite-lepidolite, K-feldspar and albite are typical rock-forming minerals; topaz is a common accessory phase, indicative of high peraluminity of this type of granites; cassiterite is present as a uniquely important tin mineral, typically rich in Nb and Ta. Mineralogical distinction between the two types of tin granites is largely controlled by redox state, volatile content and differentiation of magmatic melts. In oxidized metaluminous granitic melts, Sn4＋ is readily concentrated in Ti-bearing rock-forming and accessory minerals. Such Sn-bearing minerals are typical of oxidized tin granites, and are enriched in granites at the late fractionation stage. In relatively reduced peraluminous granitic melts, Sn2＋ is not readily incorporated into rock-forming and accessory minerals, except for cassiterite at fractionation stage of granite magma, which serves as an indicator of tin mineralization associated with this type of granites. The nature of magma and the geochemical behavior of tin in the two types of granites thus result in the formation of different types of tin deposits. Metaluminous granites host disseminated tin mineralization, and are locally related to deposits of the chlorite quartz-vein, greisen, and skarn types. Greisen, skarn, and quartz-vein tin deposits can occur related to peraluminous granites, but disseminated mineralization of cassiterite is more typical.

Mineralogical feature and geological significance of muscovites from the Longyuanba Indosinian and Yanshannian two-mica granites in the eastern Nanling Range

Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous.Particularly,over 91%of the Indosinian granites exposed in the region are peraluminous in composition.It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole,a good mineral barometer commonly identified in metaluminous granites.Muscovite is a common mineral in peraluminous granites,certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites.In this paper,results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported.Based on petrographic studies,the primary muscovite can be discriminated from the secondary muscovites.Muscovites from the Indosinian two-mica granites are enriched in Ti,Al,Mg,and Na,and depleted in Fe and Mn.Geochemically,these muscovites were considered as primary,whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams.Barometer estimations show that pressures calculated for primary muscovites are accurate,but those calculated for secondary muscovites are overestimated.The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar,corresponding to^19 km in depth,suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.

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）.

Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems

Discriminating barren and fertile intrusions is one of the main challenges in the search for rare-element pegmatites.Diagrams comprising more than one element can make discrimination of productive and barren samples more valid.These diagrams distinguish samples by simultaneous means of positive and/or negative correlations between variables.A ternary diagram for S-type peraluminous granites has been obtained in this study.Firstly,a database composed of Ta-bearing and barren granitic systems was created,then geochemical behavior of trace elements was studied,and statistical investigations were done using GCDkit software,which resulted in the Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf)ternary diagram which can distinguish the non-mineralized granites from productive ones.The Ta-bearing samples,which are situated in the fertile field in the diagram,are those which have high Nb and Ta contents,elevated Ga content and the lowest Nb/Ta and Zr/Hf values.

Zircon and Monazite Ages Constraints on Devonian Magmatism and Granulite-Facies Metamorphism in the Southern Qaidam Block:Implications for Evolution of Proto-and Paleo-Tethys in East Asia

High-temperature magma generation process and granulite-facies metamorphism can provide important information about mantle-crustal interaction and tectonic evolution. The strongly peralu- minous monzonite pluton, the Jinshuikou cordierite granite on the southern margin of the Qaidam Block, can provide important information about the mantle-crustal interaction and constraints on tectonic tran- sition from Proto-Tethys to Paleo-Tethys. This pluton develops enclaves of mafic granulite, amphibolite and quartzofeldspathic rocks, and is cut by massive monzonitic leuco-granite veins. Zircon and monazite U-Pb dating for the cordierite granite, the granulite enclaves and a massive monzonitic leuco-granite vein reveal that the cordierite granitic magma was generated from Mesoproterozoic continental crust with protolith derived from a provenanee that was composed of 〉2.8 Ga old recycled crustal materials and re- corded a -1.7 Ga magmatic event. The continental crust underwent low-pressure granulite-facies metamorphism at -380 Ma ago, whereas the cordierite granite magmas was generated and emplaced during 380 Ma, followed by intrusion of the massive monzonitic leuco-granite vein at circa 370-330 Ma. These data suggest that after the final closure of Proto-Tethys Ocean spreading along the southern Qaidam Block at -420 Ma, break-off of the subducted slab or delamination of the lower crustal base and upwelling of the asthenospheric mantle beneath the southern Qaidam Block occurred before the Mid-Devonian, and that the initiation of the Paleo-Tethys tectonics might initiate near the end of Early-Carboniferous in the East Kunlun-Qaidam region, East Asia.