Petrological features of the Santa Teresa Granitic Complex Southeastern Uruguay

The Santa Teresa Granitic Complex, located in the north-eastern region of the Rocha Department (Eastern Uruguay), is an epizonal Late-Brasiliano granite intruded in the low-grade metasedimentary sequence of the Rocha Group. Twelve different facies types, each with distinctive structural-petrographic features, were recognized during detailed mapping (1:50,000) of the central-eastern part of the granitic complex and form two magmatic suites. The Santa Teresa Calk-alkaline Suite is composed of mostly porphyritic 3a–3b granites with variable amounts of biotite, sphene, allanite, magnetite and microgranular enclaves and belongs to a middle to high potassium calk-alkaline series with high silica contents. In contrast, the Sierra de la Blanqueada Peraluminous Suite has a great variation of grain size, including 3a–3b granitic facies with variable content of muscovite, biotite, tourmaline, ilmenite and monazite. Zircon morphology was studied in both suites and also shows their calk-alkaline and peraluminous nature. The Santa Teresa Calk-alkaline Suite had a Late- to Post-orogenic setting whereas the Sierra de la Blanqueada Peraluminous Suite was formed during the crustal thickening related to a syn-collisional environment.     

Peraluminous leucogranites of the Cordilheira Suite: A record of Neoproterozoic collision and the generation of the Pelotas Batholith,Dom Feliciano Belt,Southern Brazil

The Cordilheira Suite is comprised of peraluminous granites that constitute, together with the Quitéria granite, the beginning of the formation of the Pelotas Batholith in the central portion of the Dom Feliciano Belt. The batholith is composed of seven granitic suites with minor occurrences of gabbro, diorite and subvolcanic rocks. Its evolution between 650 and 550 million years ago is characterised by a ranging from metaluminous to peraluminous rocks and from calc-alkaline to alkaline and peralkaline series. The Cordilheira Suite is composed of the Cordilheira, Arroio Francisquinho, Butiá and Três Figueiras granites, which contain muscovite and/or biotite, with the following accessory minerals: tourmaline, garnet, sillimanite, apatite, zircon, monazite and ilmenite. The granitic bodies are elongate; their ascent and emplacement were controlled by high-angle shear zones oriented at N45-70°E. They have mylonitic structures with magmatic foliation accompanied by a low-angle stretching lineation, indicating that these bodies were emplaced under syn-kinematic conditions during a transcurrent event. The granites have high-K calc-alkaline affinity and are peraluminous. The LILE and REE contents are low. The K₂O/Na₂O and CaO/Na₂O ratios are approximately 1 and less than 0.3, respectively. Pseudosections calculated using the Perple X program suggests that the granites were formed at partial melting temperatures between 740 and 820 °C and pressure between 8.5 and 9 kbar. Petrographic and chemical data suggest that the magmas were generated by the partial melting of the migmatitic pelitic gneisses of the Várzea do Capivarita Complex and, to a lesser degree, orthogneisses of the Arroio dos Ratos Complex, which left a granulitic residue. UHT conditions of granulite facies metamorphism were recorded in the Várzea do Capivarita paragneisses by mineral paragenesis and indicate that temperatures between 900 and 1000 °C and pressures between 4 and 8 kbar were required for the partial melting. It is likely that these medium-pressure conditions resulted from the thickening of the continental crust when the Rio de La Plata Craton collided with the Kalahari Craton to form southwestern Gondwana at the end of the Neoproterozoic.     

喜马拉雅碰撞造山带新生代地壳深熔作用与淡色花岗岩

自从印度-欧亚大陆碰撞以来,伴随着构造演化和温度-压力-成分(P-T-X)的变化,喜马拉雅造山带中下地壳变质岩发生不同类型的部分熔融反应,形成性质各异的过铝质花岗岩。这些花岗岩在形成时代、矿物组成、全岩元素和放射性同位素地球化学特征上都表现出巨大的差异性。始新世构造岩浆作用形成高Sr/Y二云母花岗岩和演化程度较高的淡色花岗岩和淡色花岗玢岩,它们具有相似的Sr-Nd同位素组成,是碰撞早期增厚下地壳部分熔融的产物。渐新世淡色花岗岩主要为演化程度较高的淡色花岗岩,可能指示了喜马拉雅造山带的快速剥露作用起始于渐新世。早中新世以来的淡色花岗岩是喜马拉雅造山带淡色花岗岩的主体,是变泥质岩部分熔融的产物,包含两类部分熔融作用——水致白云母部分熔融作用(A类)和白云母脱水熔融作用(B类)。这两类部分熔融作用形成的花岗质熔体在元素和同位素地球化学特征上都表现出明显的差异性,主要受控于两类部分熔融作用过程中主要造岩矿物和副矿物的溶解行为。这些不同期次的地壳深熔作用都伴随着高分异淡色花岗岩,伴随着关键金属元素(Nb、Ta、Sn、Be等)的富集,是未来矿产勘探的重要靶区。新的观测结果表明:在碰撞造山带中,花岗岩岩石学和地球化学性质的变化是深部地壳物质对构造过程响应的结果,是深入理解碰撞造山带深部地壳物理和化学行为的重要岩石探针。     

The late- to postorogenic transition in the Neoproterozoic Agudos Grandes Granite Batholith (Apiaí Domain,SE Brazil): Constraints from geology,mineralogy, and U–Pb geochronology

The latest evolution of the Neoproterozoic Agudos Grandes Batholith (Apiaí domain, SE Brazil) is marked by an important change in the type of granitic magmatism. The “late-orogenic” Piedade, Roseira, Serra dos Lopes, and Pilar do Sul granites are elliptical plutons with roughly concentric zoning and a spatial arrangement suggesting a continuous southwestward migration of the magmatic focus. The main rock types are “contaminated” calc-alkaline granites that range from mafic-rich (color index > 10), porphyritic biotite (±muscovite) granite-granodiorite in Piedade to pink, equigranular, muscovite–biotite leucogranite (CI < 5) strongly affected by hydrothermal effects in Pilar do Sul. U–Pb monazite dating indicates that these plutons were emplaced during 600–605 Ma, slightly after the main “synorogenic” magmatic stage (615–610 Ma), which was dominated by high-K, calc-alkaline, metaluminous, porphyritic hornblende-biotite granites with minor peraluminous leucogranite bodies. The “postorogenic” granites are divided into two groups on the basis of pluton shapes and U–Pb dating, both with “A-type” affinities. The approximately 585 Ma group (São Miguel Arcanjo and Capão Bonito granites) relates to the Itu granitic province, which developed around 10 m.y. after the cessation of the main regional compressional events, and cross-cuts the reworked border of the Paranapanema plate; the younger, approximately 565 Ma group is represented by two elongated plutons (Serra da Batéia and Serra da Queimada) that seem to reflect coeval orogenic events farther east in the Ribeira belt. The modal composition, magnetic susceptibility, and mafic mineral chemistry of the late-orogenic granites are consistent with an origin by contamination of metaluminous, oxidized, calc-alkaline magmas with crustal melts.     

阿尔金造山带南缘岩浆混合作用:玉苏普阿勒克塔格岩体岩石学和地球化学证据

阿尔金造山带南缘玉苏普阿勒克塔格岩体中的似斑状中粗粒黑云钾长花岗岩发育有岩浆成因的暗色包体,并且该花岗岩被花岗细晶岩呈脉状侵入。该岩体含有丰富的岩浆混合作用特征: 如暗色包体中的碱性长石斑晶、针状磷灰石、长石的环斑结构、石英/斜长石主晶和榍石眼斑等。暗色包体、寄主花岗岩和花岗细晶岩代表了岩浆混合演化过程中不同端元比例混合的产物。地球化学特征上,钾长花岗岩和暗色包体的主要氧化物含量在Harker图解中多呈线性变化。暗色包体主要为闪长质,MgO、K₂O含量高,为钾玄岩系列,总体上高场强元素不亏损,显示了岩浆混合中的基性端元信息,可能为幔源熔体结晶分异或壳幔物质的混合产物。寄主花岗岩均为准铝质,富碱,为高钾钙碱性系列,亏损Nb、Ta、Sr、P、Ti等高场强元素,高K₂O/Na₂O,富集高不相容元素,Ga含量高,显示了A型花岗岩的特征,Th/U 和Nb/Ta比值分别介于为6.67~10.96、8.99~11.94,代表了下地壳源区。花岗细晶岩均为钠质、过铝质,TiO₂、MgO含量低, Na₂O和CaO含量高,具有混合岩浆侵位后分异的特征。岩相学和地球化学特征说明岩浆混合作用对于环斑结构花岗岩的形成起到重要作用。花岗细晶岩中环斑长石的斜长石外环与钾长石内核的厚度比大于钾长花岗岩中的环斑长石,指示混合岩浆在一定的减压条件下更有利于环斑结构的形成。玉苏普阿勒克塔格岩体中的钾玄质暗色包体、高钾钙碱性花岗岩和中钾钙碱性花岗细晶岩代表了岩浆演化不同阶段的产物,反映了一个幔源岩浆和下地壳不断相互作用,引起地壳连续伸展减薄的过程,指示阿尔金南缘在早古生代末期存在造山后伸展背景下的幔源岩浆底侵作用。同一岩体中两种不同时代岩性的环斑结构显示了该岩体形成历史中的一定时空演化关系,代表了伸展过程中不同阶段的产物。     

Elemental and boron isotopic variations in tourmaline in two-mica granite from the Cuona area,Tibet: Insights into the evolution of leucogranitic melt

Two-mica granite is the most common magmatic rock type in the Himalayan leucogranite belt, which has close relationship with rare metal mineralization. Its genesis is generally attributed to magmatic differentiation. In recent years, the mineral geochemical compositions are increasingly used to study magmatic differentiation, which are significant for deciphering the melt evolution and element migration processes. In this study, in-situ major and trace element and boron isotope compositions for tourmalines from two-mica granites in the Cuona and Cuonadong leucogranites in the Cuona area are conducted to determine microscopic changes in mineral assemblages and geochemical compositions. Analytical results show that the tourmalines in the Cuonadong leucogranite were crystallized earlier relative to the tourmalines in the Cuona leucogranite during magmatic differentiation. The volatile contents have a genetic relationship with incompatible elements in tourmaline, which is possibly responsible for the formation of tourmaline zonation and the enrichment of Sr, Zn, and Pb during magmatic differentiation. The B isotopic composition of tourmaline in the Cuona area suggests that the granitic magma was dominantly derived from the partial melting of the metasedimentary source rocks. Their B isotope variations likely resulted from fluid exsolution during B-rich melt evolution. High rare metal contents in tourmalines indicate that the two-mica granites in the Cuona area may have great mineralization potential.     

Provenance,tectonic setting and source of Archean metasedimentary rocks of the Browns Range Metamorphics,Tanami Region,Western Australia

This study combines U–Pb age and Lu–Hf isotope data for magmatic and detrital zircons, with whole-rock geochemistry of the Browns Range Metamorphics (BRM), Western Australia. The BRM are medium- to coarse-grained metasandstones that consist of angular to sub-rounded detrital quartz and feldspars with minor granitic lithic fragments. The sequence has undergone partial to extensive quartz–muscovite alteration and rare-earth-element mineralisation and has been intruded by mafic/ultramafic, syenitic and pegmatitic intrusive rock units. Uranium–Pb and Lu–Hf isotopic data on detrital zircons from the metasandstones and intruding granitic rocks yield a well-defined age of ca 3.2 to ca 3.0 Ga for all samples, with relatively radiogenic ?_Hf values (?_Hf = –1.7 to 5.1) indicating derivation from Mesoarchean granite basement of juvenile origin. This is consistent with geochemical and petrological data that support deposition from a granitic source in a continental rift basin setting. The timing of sediment deposition is constrained between the ca 3.0 Ga age of the source rocks and ca 2.5 Ga age of the granitic intrusive bodies that cross-cut the metasedimentary rocks. The ca 2.5 Ga zircons from the intrusive rocks have ?_Hf model ages of ca 3.4 to ca 3.1 Ga, which is consistent with formation via partial melting of the BRM, or the Mesoarchean granite basement. Zircons of the Gardiner Sandstone that unconformably overlies the BRM return detrital ages of ca 2.6 to ca 1.8 Ga with no trace of ca 3.1 Ga zircons, which discounts a significant contribution from the underlying BRM. The Mesoarchean age and isotopic signatures of the BRM zircons are shared by some zircon records from the Pine Creek Orogen, and the Pilbara, Yilgarn and Gawler cratons. Collectively, these records indicate that juvenile Mesoarchean crust is a more significant component of Australian cratons than is currently recognised. This work also further demonstrates that detrital minerals in Paleoproterozoic/Archean sedimentary rocks are archives to study the early crustal record of Earth.     

Constraints of Mantle and Crustal Sources Interaction during Orogenesis of Pre- and Post-collision Granitoids from the Northern Arabian-Nubian Shield: A Case Study from Wadi El-Akhder Granitoids,Southern Sinai,Egypt

The Egyptian older and younger granitic rocks emplaced during pre- and post-collision stages of Neoproterozoic Pan-African orogeny, respectively, are widely distributed in the southern Sinai Peninsula, constituting 70% of the basement outcrops. The Wadi El-Akhder, southwestern Sinai, is a mountainous terrain exposing two granitoid suites, namely the Wadi El-Akhder Older Granites (AOG) and the Homra Younger Granites (HYG). The AOG (granodiorites with subordinate tonalite compositions) have geochemical characteristics of medium-K calc-alkaline, metaluminous to mildly peraluminous granitoids formed in an island-arc environment, which are conformable with well-known Egyptian older granitoids rocks, whereas the HYG display calc-alkaline to slightly alkaline nature, peraluminous syeno-, monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites. With respect to the AOG granitoids, the HYG granites contain lower Al2O3, FeO*, MgO, MnO, CaO, TiO2, Sr, Ba, and V, but higher Na2O, K2O, Nb, Zr, Th, and Rb. The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). The geochemical features of the AOG follow assimilation-fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source. The chemical characteristics of the AOG are remarkably similar to those of subduction-related granitoids from the Arabian-Nubian Shield (ANS). The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments, which is similar to the most of younger granitic suites in the ANS.     

Age and isotopic studies of some Pan-African granites from North-Central Nigeria

Twenty-nine Rb-Sr whole-rock isotopic analyses and three U-Pb zircon analyses on foliated granites and largely unfoliated charnockitic rocks indicate that the central part of the Pan-African belt in west Africa was characterised by intense orogenic plutonism. These data and Rb-Sr analyses on muscovite books from late cross-cutting pegmatites indicate that the peak of magmatic activity occurred 610 ± 10 m.y. ago.Initial ⁸⁷Sr/⁸⁶Sr ratios for the granitic and charnockitic rocks are in the range 0.7065–0.7125, and indicate a significantly older crustal component in the magmas.     

The origin of the acidic and basic rocks of the Tichka Massif,Morocco, based on rare earth elements

The Tichka Massif is one of many late-Paleozoic massifs emplaced in northwestern Africa. It consists of granitic pods surrounded by a complex assemblage of dioritic and gabbroic rocks. Previous workers have suggested an independent origin for the granitic and gabbroic rocks and this study evaluates this suggestion using REE data.The Ce/Yb ratio versus Ce plot for the gabbro shows a large Ce/Yb variation for small changes in Ce, whereas the granites show a linear increase of Ce/Yb with Ce. In the granites, the samples with the highest Ce/Yb ratio have the lowest SiO₂, whereas in the gabbros the Ce/Yb ratio is lowest in the low SiO₂ rocks. The diorites show a tightly clustered pattern.Based on the REE data the granite-diorite-gabbro sequence cannot be related by fractional crystallization. Partial melting of a garnet bearing mantle is consistent with the gabbroic trends. Partial melting of a crustal source with small amounts of apatite and abundant hornblende can produce the granitic rocks.     

Geochemistry and geochronology of the rapakivi granites and associated rocks in the midwest portion of the Serra da Providência composite batholith,SW of Amazonian craton,Rondônia,Brazil

The Serra da Providência batholith includes the type area of the homonymous suite, the oldest rapakivi magmatic assemblage in the SW of the Amazonian craton (1.60–1.53 Ga). In the midwest portion of this massif, besides wiborgites/pyterlites and granophyric syenogranites, a leucosyenogranite facies and porphyritic rhyolites constitute new rock varieties recently described in that area. UPb LA-MC-ICP-MS zircon ages of 1574 ± 9 Ma and 1604 ± 3 Ma, respectively, were obtained for these new varieties and confirm their link with the Serra da Providência magmatism, whereas the subvolcanic rocks are older than the main rock varieties and were formed in a precursor event. These granitic facies are metaluminous to peraluminous, alkali-calcic, A2-type, ferroan granites. Their FeOt/(FeOt + MgO) ratios vary from 0.83 to 0.98 and suggest that these rocks were crystallized from oxidized-to reduced-A-type magmas, where the leucosyenogranites and granophyric sienogranites tend to be formed under more reduced conditions. They show fractionated REE patterns with very pronounced to weak negative Eu anomalies. The presence of granophyric textures and miarolitic cavities in equigranular syenogranitic facies suggests that these rocks were formed at shallow crustal depths, lower than 3 km. Three samples of leucosyenogranite have silica contents higher than 75% and low K/Rb ratios (<150), similarly to the tin specialized granites described in the Amazonian craton. Two distinctive groups of mafic rocks were recognized in the study area: porphyritic and equigranular gabbronorites. They correspond to tholeiitic basalts, with #Mg varying from 37 to 41 in porphyritic gabbronorites and 51 to 65 in equigranular gabbronorites. The low to moderate #Mg suggests that these rocks were crystallized from more evolved basaltic magmas. The porphyritic gabbronorites are enriched in TiO₂, FeOt, K₂O, P₂O₅ and REE compared to the equigranular gabbronorites that are enriched in MgO and CaO. The porphyritic gabbronorites have significant negative Eu anomalies a feature not observed in the equigranular gabbronorites. Porphyritic gabbronorites geochemical characteristics are similar to calc-alkaline basalts, whereas equigranular gabbronorites are similar to continental basalts. Petrographic, geochemical, and geological data of the felsic facies and the presence of associated mafic rocks corroborate the bimodal and post-collisional character of this magmatism. The occurrence of porphyritic rhyolites associated with shallow level plutonic granites in the Serra da Providência batholith reinforces the similarities between the Rondonian granites and the classical Fennoscandian rapakivi granites.     

湘南王仙岭花岗岩体的锆石U-Pb年代学、地球化学、锆石Hf同位素特征及其地质意义

湘南王仙岭岩体由主体电气石黑云母花岗岩和侵入其内部的黑云母二长花岗岩组成,LA-MC-ICPMS锆石U-Pb定年显示电气石黑云母花岗岩形成于印支期(235.0±1.3Ma),黑云母二长花岗岩形成于燕山期(155.9±1.0Ma),表明该岩体是两期岩浆活动的产物。这两期岩石均为高钾钙碱性系列,A/CNK值为1.07～1.66,属过铝-强过铝质花岗岩类。稀土元素显示LREE富集,HREE亏损,Eu负异常明显(0.01～0.38)的特征。早期电气石黑云母花岗岩和晚期黑云母二长花岗岩的εHf(t)值分别为-7.92～+4.61和-10.66～-5.35;两阶段Hf模式年龄(tDM2)分别为1758～967Ma和1875～1538Ma。两期花岗岩均来自于古中元古代地壳物质重熔,其中早期电气石黑云母花岗岩在侵位上升过程中捕获了部分幔源老锆石,成岩过程中有少量地幔物质参与,且其源区具有高εHf(t)值的特点。综合前人研究成果,本文认为华南中生代印支期和燕山期均有钨锡矿化作用,印支期花岗质岩浆形成于碰撞挤压作用间隙伸展环境,而燕山期花岗质岩浆可能形成于大陆边缘弧后伸展环境。     

The Late Neoproterozoic granitoid magmatism of the Pelotas Batholith, southern Brazil

Geological mapping, petrography, geochemistry, and isotope studies enable the division of the Pelotas Batholith into six granitic suites: Pinheiro Machado (PMS), Erval (ES), Viamão (VS), Encruzilhada do Sul (ESS), Cordilheira (CS), and Dom Feliciano (DFS). The rocks of the PMS show a large compositional range (granite through granodiorite to tonalite), and the suite is considered pre- to syncollisional. Other suites show restricted compositional variations (granite to granodiorite) and are late to postcollisional. In general, the suites are metaluminous to slightly peraluminous (PMS, ES, and VS) or peraluminous (CS) or have alkaline tendencies (ESS and DFS). The magmatic evolution corresponds to high-K calc-alkaline to alkaline magmatism. The suites are enriched in K, Rb, and REE compared with rocks of typical calc-alkaline series. Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.705 to 0.716, except in the CS, where they attain values of 0.732–0.740. Sm–Nd T_DM model ages vary between 0.98 and 2.0 Ga, with initial ε_Nd values ranging from −0.3 to −10. U–Pb zircon dates of samples from PMS, VS, and ESS suggest an age between 0.63 and 0.59 Ga for magmatism. Rb–Sr dates of samples of alkaline granites from DFS present ages between 0.57 and 0.55 Ga. The main tectonic controls of the magmatism of the Pelotas Batholith are high-dip sinistral shear zones.     

Geochemical Characterization of Granites of Distinctly Different Ages in the Alto Pajeu Terrane,Borborema Province,Northeast Brazil

This work presents geochemical data for granitic plutons that intruded the Alto Pajeu terrane, Borborema Province, northeastern Brazil, during several distinct tectonic cycles. The Alto Pajeu terrane is represented in the study area by metasedimentary (São Caetano Formation) and metavolcanosedimentary (Irajai Formation) sequences overlying a gneissic-migmatitic basement. Several granitic plutons of different ages, mineralogical proportions, and chemical compositions have invaded the supracrustal and basement rocks. Two deformational events, one tangential (older) and the other transcurrent (younger), were identified. The metamorphic grade determined for the metasedimentary rocks varies from amphibolite to high amphibolite facies, with temperatures ranging from 600 to 680°C and pressures from 6.0 to 7.5 kbar. Three magmatic events with distinct ages were identified for the tectonic evolution of the Alto Pajeu terrane in the study area. The older magmatic event is related to the Transamazonico orogenic cycle (～ 2.0 Ga), the intermediary magmatic event to the Cariris Velhos cycle (～0.97 Ga), and the younger magmatic event to the Brasiliano orogenic cycle (～0.55 Ga). The Cariris Velhos- and Brasiliano-age granites are geochemically classified as peraluminous, subalkalic granites of the calc-alkalic series, whereas the Transamazônico-age granite is metaluminous and alkaline. Geochemical data presented and discussed here suggest that the magmatism related to the three tectonic events had distinctly different sources and petrogenetic evolutions.     

藏南特提斯喜马拉雅带始新世隆子-恰嘎次火山岩区:雅拉香波二云母花岗岩的高位岩浆体系

已有的数据表明,大约在中始新世(44～40Ma),西藏特提斯喜马拉雅带(雅拉香波穹窿地区)经历了一次特别的地壳深熔作用,产生了大量高Sr/Y比值的二云母花岗岩.在雅拉香波穹窿南部的隆子-恰嘎地区,发育一套流纹质次火山岩,以小岩体或岩脉形式侵位于侏罗纪日当组的砂岩和页岩之中.岩相学观察、锆石U-Pb年代学、全岩元素和同住素(Sr、Nd)地球化学数据表明:(1)该次火山岩形成于约43～41Ma,与北部二云母花岗岩体的形成年龄相似;(2)该次火山岩经历了强烈的岩浆演化后期的岩浆-热液作用和钙长石分离结晶作用,导致该套岩浆岩强烈的Eu负异常、明显降低的Sr含量和锆石岩浆增生边的普通Pb和U浓度明显升高;和(3)该套次火山岩和二云母花岗岩属于同一岩浆过程的不同的构造层次,在时间、空间和成岩物质来源上具有一致性,同属于中始新世青藏高原主碰撞时中下地壳部分熔融的产物.     

Petrology and geochemistry of Jura granite and granite gneiss in the Neelum Valley,Lesser Himalayas (Kashmir,Pakistan)

Late Proterozoic rocks of Tanol Formation in the Lesser Himalayas of Neelum Valley area are largely green schist to amphibolite facies rocks intruded by early Cambrian Jura granite gneiss and Jura granite representing Pan-African orogeny event in the area. These rocks are further intruded by pegmatites of acidic composition, aplites, and dolerite dykes. Based on field observations, texture, and petrographic character, three different categories of granite gneiss (i.e., highly porphyritic, coarse-grained two micas granite gneiss, medium-grained two micas granite gneiss, and leucocratic tourmaline-bearing muscovite granite gneiss), and granites (i.e., highly porphyritic coarse-grained two micas granite, medium-grained two micas granite, and leucocratic tourmaline-bearing coarse-grained muscovite granite) were classified. Thin section studies show that granite gneiss and granite are formed due to fractional crystallization, as revealed by zoning in plagioclase. The Al saturation index indicates that granite gneiss and granite are strongly peraluminous and S-type. Geochemical analysis shows that all granite gneisses are magnesian except one which is ferroan whereas all granites are ferroan except one which is magnesian. The CaO/Na₂O ratio (>0.3) indicates that granitic melt of Jura granite gneiss and granite is pelite-psammite derived peraluminous granitic melt formed due to partial melting of Tanol Formation. The rare earth element (REE) patterns of the Jura granite and Jura granite gneiss indicate that granitic magma of Jura granite and Jura granite gneiss is formed due to partial melting of rocks that are similar in composition to that of upper continental crust.     

藏南冈底斯岩基日多地区花岗岩体形成时代和地球化学特征

冈底斯岩基广泛发育三叠纪-中新世的岩浆岩,是研究与新特提斯洋北向俯冲和印度-欧亚大陆碰撞相关的构造岩浆作用特征的天然实验室。日多地区花岗岩体位于藏南墨竹工卡县东侧日多乡附近,其主体为花岗岩,被花岗闪长玢岩脉侵入。锆石U-Pb地质年代学表明:主体花岗岩形成于62.7±0.5Ma,侵入其中的花岗闪长玢岩脉形成于59.5±1.5Ma,并捕获了大量的侏罗纪岩浆岩锆石(155.4±1.8Ma)。日多地区花岗岩体的全岩地球化学特征为:(1)高Si O_2、Na_2O、Al_2O_3,低Fe O~T、MgO、Ti O_2;(2)富集轻稀土(LREE),亏损重稀土(HREE)及高场强元素Nb、Ta、Ti、P元素;(3)具有Eu负异常,总体显示高钾钙碱性、过铝质花岗岩和岛弧型岩浆岩特征。锆石Hf同位素特征暗示其岩浆源区为基性下地壳物质。花岗闪长玢岩脉裹挟大量侏罗纪岩浆型锆石,表明冈底斯岩基拉萨以东地区可能经历了较广泛的晚侏罗世岩浆作用。     

Rhyacian and Neoproterozoic magmatic associations of the Gurupi Belt,Brazil: Implications for the tectonic evolution,and regional correlations

The Gurupi Belt, in north-northeastern Brazil, is a mobile belt developed in the south-southwestern margin of the São Luís cratonic fragment and crops out as a tectonic and erosional window within the Phanerozoic cover. Field, petrographic, geochemical, geochronological, and Nd isotopic information (new and published) constrain the timing and types of magmatic associations present in the belt and the tectonic settings in which they formed. The Rhyacian was the main period of magmatic activity, which can be grouped into two main stages. (1) ~2185–2130 ​Ma: pre-collisional, juvenile, calc-alkaline magnesian and calcic ferroan granitoid suites, and minor calc-alkaline and tholeiitic mafic plutonism (now amphibolites), formed in intra-oceanic to transitional/continental arcs; and intra- or back-arc volcano-sedimentary basin. (2) ~2125–2070 ​Ma: syn- (two-mica granites) to late-collisional (potassic to shoshonitic granites and quartz-syenite) plutonic suites produced after crustal thickening and melting, with localized migmatization, that intruded during the compressive D₁ deformational phase and concomitantly with greenschist to amphibolite metamorphism. There is a zonation of the Rhyacian episodes, with intra-oceanic stages occurring to the northeast, and the continental arc and collisional phases occurring to the southwest, indicating the presence of an active continental margin to the southwest, and subduction from NE to SW (present-day configuration). This magmatic framework is a continuation to the south of what is described for the São Luís cratonic fragment to the north, and the orogenic scenario is identical to what is observed for the same period in the West African Craton (Eburnean/Birrimian orogen), which additionally supports previous geological correlations. In the Neoproterozoic, a few magmatic occurrences are recognized. An extensional event allowed the intrusion of an anorogenic, nepheline syenite at ca. 730 Ma, which was followed by the intrusion of a crustal, calc-alkaline microtonalite, of uncertain tectonic setting, at 624 ​Ma. Both intrusions underwent greenschist to amphibolite facies metamorphism between 580 Ma and 529 ​Ma. This metamorphic event is probably related to crustal thickening, which produced crustal melting and intrusion of two-mica granites between 595 Ma and 549 ​Ma. The absence of oceanic and arc-related assemblages, along with geophysical information about the basement of the Phanerozoic cover indicates an intracontinental setting for the Neoproterozoic–Early Cambrian evolution of the Gurupi Belt, with rifting and posterior closure of the basin, without oceanization. Rifting and closure correlate in time with the onset of Rodinia breakup and West Gondwana assembly, respectively, but we interpret the events in the Gurupi Belt as having no direct role in these two global supercontinent-related events, but, instead, as being related to orogenic events occurring in the periphery of the West African and Amazonian cratons at that time.     

西藏冈底斯中段桑桑花岗质岩体锆石U-Pb年龄与Hf同位素组成及其对岩石成因和构造演化的制约

以西藏冈底斯中段西侧桑桑花岗质岩体为对象,进行了系统的年代学、元素地球化学和锆石Hf同位素组成研究,据此阐明了岩体成因,并探讨了其构造意义。锆石LA-ICP-MS U-Pb定年表明,桑桑花岗质岩体的成岩年龄为49~54 Ma。化学组成上,岩体具有亚碱、准铝、贫磷的特征(A/NKC1.10,P_2O_50.20%),属钙碱性I型花岗岩类。岩体富Cs、Rb、Ba、Th、U、K、Pb和轻稀土,贫Nb、Ta、P与Ti,表现出弧岩浆岩的地球化学特征。岩体的锆石εHf(t)值变化较大,散布于正值与负值之间(=-4.24~+5.49),指示其形成存在不同来源物质的贡献。综合分析表明,桑桑花岗质岩体起源于初生地壳的部分熔融,但在成岩过程中有古老地壳组分的参与。结合区域地质背景,笔者认为这一古老地壳组分最可能来自印亚碰撞过程中俯冲下插的印度地壳,由此说明印度-欧亚大陆碰撞的起始时间应早于54 Ma。     

Petrology and geochemistry of Mesozoic granitic rocks from the Nansha micro-block, the South China Sea: Constraints on the basement nature

There are several micro-blocks dispersed in the South China Sea (SCS), e.g., Xisha-Zhongsha block, Nansha block and Reed-Northeastern Palawan block, etc., but detailed petrological constraints on their basement nature were previously lacking. The magmatic ages for granitic rock samples from two dredge stations in the Nansha micro-block vary from 159 to 127 Ma, which are comparable to magmatic activities occurred in the northern margin (Pearl river mouth), HongKong and East China. Petrographic characteristics, major-, trace element and Sr–Nd isotopic data of nine samples from two dredged station performed in the Nansha micro-block, the SCS, are reported. Petrographically, these granitic rocks can be divided into two groups which underwent a complex history of magmatic process, i.e., tonalitic rock (Group I) and monzogranitic rock (Group II). The Rittmann index (σ) for these rocks (1.9–3.1) suggest that they belong to calc-alkaline rocks. Group I rocks which is of typical I-type, have higher contents of TiO₂, Al₂O₃, FeO, MgO, CaO, Na₂O and P₂O₅, but lower values of SiO₂ and K₂O, when compared with those of Group II with I-type characteristics. Group I rocks are produced by partial melting of older Precambrian basement with the variable influence of mantle-derived magma which results from the interaction of released fluids from the subducted slab and the overlying mantle wedge in a general convergent margin setting, and Group II rocks result from partial melting of lower crustal basic rocks (amphibolite) and/or further partial melting of the Group I rocks associated with the variable influence from the underplating mantle-derived magma resulting from lithospheric extensional regime. Both Groups I and II have undergone assimilation and fractional crystallization (AFC) processes during its petrogenesis. This study therefore demonstrates that there exists a continental basement within micro-blocks in the South China Sea, and further supports the idea that a Middle Jurassic to Mid-Cretaceous subduction zone existed across the temporary Taiwan, Palawan to Southern Vietnam, which was associated with westward to northestward convergence of the Pacific Plate during Late Mesozoic. We suggest that this subduction zone may have been connected with the paleo-Pacific plate subduction zone offshore eastern China during Mesozoic era. This study provides petrologic data for the pre-Cenozoic tectonic evolution of the South China Sea.