Geochemical Characteristics Of A-type Granites Research Articles

Early Paleoproterozoic tectonic evolution of the Yinshan Block in the North China Craton: Constraints from the geochronology and geochemistry of basic to felsic magmatic rocks in the Guyang area

The earliest Paleoproterozoic magmatic rocks have recorded the transition from the Neoarchean cratonization of the NCC to its post-Archean tectonic evolution, which will provide insight into the onset of the modern-style subduction. This study focuses on the early Paleoproterozoic granodiorite, monzonitic diorite, alkali feldspar granite and gabbro in the Guyang area to decipher the early Paleoproterozoic tectonic evolution of the Yinshan Block. Zircon U–Pb dating indicates that these rocks were emplaced at ca 2.46 – 2.41 Ga. The ∼ 2.46 Ga granodiorite samples have high Sr contents, low Y and Yb contents with high Sr/Y ratios, similar to the characteristics of adakitic rocks. Based on their high Mg# values, Na2O/K2O ratios, Cr, Co and Ni concentrations, and positive εNd(t) values, we propose that the granodiorite was likely produced by partial melting of subducting oceanic slab. The contemporary monzonitic diorite samples are characterized by high Nb concentration, which are similar to those of Nb-enriched basalt/andesite. The monzonitic diorite samples are characterized by LREE-enriched patterns with negative Eu anomalies and have negative εNd(t) values (-0.4 – −1.4), which suggest that they were derived from partial melting of fertile mantle peridotite metasomatized by adakitic melts. The ∼ 2.44 Ga alkali feldspar granite samples have negative εNd(t) values (-4.0 – −1.9) and show geochemical characteristics of A-type granites, indicating an extension background. The distinct geochemical characteristics of ca. 2.46 – 2.44 Ga Guyang adakatic granodiorite, Nb-enriched monzonic diorite and A-type alkali feldspar granite suggest that a slab window was opened in the southern margin of the Yinshan Block. The gabbros were emplaced at ∼ 2.41 Ga revealed by U-Pb dating. The enriched LREEs and relatively flat HREEs patterns with negative Nb-Ta anomalies of gabbros indicate a subducting tectonic setting. Their high Ba/Th and low Nb/Y ratios imply that the mantle source was metasomatized by slab-derived fluids. This special rock association not only constrains the tectonic evolution in the southern margin of the Yinshan Block, but also indicates modern-style subduction operated already in the early Paleoproterozoic.

Geochronology, geochemistry and tectonic implications of late Carboniferous Daheyan intrusions from the Bogda Mountains, eastern Tianshan

AbstractThe Daheyan region, situated in the SW of the Bogda Mountains in eastern Tianshan, is important for understanding the accretionary history of the Central Asian Orogenic Belt. We investigated Carboniferous intrusions from the Daheyan area, SW Bogda Mountains, obtaining new zircon U–Pb ages, whole-rock geochemical data and Hf isotope data for these intrusions. Zircon U–Pb dating indicates that syenogranite, diorite, granodiorite and monzonite of the Daheyan intrusions were all formed during late Carboniferous (311–303 Ma) magmatism. The syenogranite has geochemical characteristics of A-type granites that were mainly sourced from melting of juvenile crust. In comparison, the low-Mg-number diorite intrusion, with tholeiite and metaluminous features, was derived from young crust and mixed some mantle materials. The granodiorite and monzonite are both I-type granites, and are both sourced from the melting of juvenile crust. Based on a comprehensive analysis of previous geochronological, geochemical and isotopic data of magmatic and sedimentary rocks in the Bogda–Harlik belt, we consider that late Carboniferous intrusive rocks of the Bogda Mountains formed in an intra-arc extension related to a continent-based arc setting.

He, Ar, and S isotopic constraints on the relationship between A-type granites and tin mineralization: A case study of tin deposits in the Tengchong–Lianghe tin belt, southwest China

The Tengchong–Lianghe tin belt in western Yunnan Province, southwest China, is the most important tin mineralization belt in the Sanjiang Tethyan Metallogenic Domain (STMD). There are two large tin deposits in this belt: the Paleogene Lailishan and the Late Cretaceous Xiaolonghe deposits. The two deposits are spatially and temporally associated with the Lailishan and Xiaolonghe granitic plutons, respectively. Recent studies suggest that the Lailishan and Xiaolonghe granitic plutons exhibit most of the mineralogical and geochemical characteristics of A-type granites. In this study, we discuss the He, Ar, and S isotopes of pyrite samples from the two large tin deposits to trace ore-forming fluids and elements. We also review the relationship between A-type granites and tin deposits from a new perspective. The δ34SCDT values of the Lailishan tin deposit range from +4.9‰ to +6.7‰, with an average value of +5.53‰. The δ34SCDT values of the Xiaolonghe tin deposit range from +5.0‰ to +8.1‰, with an average value of +6.33‰. These values are slightly higher than those of the granites (0‰ to +5.7‰) in the Tengchong–Lianghe area, indicating mainly magmatic sources for the sulfur of ore-forming fluids with a small amount of S from the wall rocks. The Lailishan tin deposit has 3He/4He values of 1.57–3.46 Ra, with an average value of 2.078 Ra, and 40Ar/36Ar values of 382.00–622.99, with an average value of 459.67. The Xiaolonghe tin deposit has 3He/4He values of 0.53–0.88 Ra, with an average value of 0.686 Ra, and 40Ar/36Ar values of 301.06–348.43, with an average value of 322.04. These values suggest that ore-forming fluids of the Lailishan and Xiaolonghe tin deposits have mixed crustal, mantle, and a small volume of meteoric water sources in different proportions. The mantle 4He values and 3He/4He values of the Lailihsan tin deposit (26–44%, 1.57–3.46 Ra) are markedly higher than those of the Xiaolonghe tin deposit (8–15%, 0.53–0.88 Ra), implying increased crust–mantle interaction in the Paleogene relative to the Late Cretaceous in western Yunnan. This interaction may be attributed to the upwelling of the asthenosphere through the break-off of the Neo-Tethyan slab during the main collisional period (65–41 Ma) of India–Asia.

The formation of the Late Cretaceous Xishan Sn–W deposit, South China: Geochronological and geochemical perspectives

The Xishan Sn–W deposit is spatially related to K-feldspar granites in the Yangchun basin, western Guangdong Province, South China. LA-ICP-MS zircon U–Pb dating for the Xishan pluton defines an emplacement age of ~79Ma (78.1±0.9Ma; 79.0±1.2Ma; 79.3±0.8Ma), consistent with the mineralization age of the Xishan Sn–W deposit constrained by molybdenite Re–Os isochron age (79.4±4.5Ma) and LA-ICP-MS cassiterite U–Pb ages (78.1±0.9Ma and 79.0±1.2Ma) for the cassiterite-quartz vein. These indicate a close genetic relationship between the granite and Sn–W mineralization. The Xishan K-feldspar granites have geochemical characteristics of A-type granites, e.g., high total alkali (Na2O+K2O=7.88–10.07wt.%), high Ga/Al ratios (10000*Ga/Al>2.6) and high Zr+Nb+Ce+Y concentrations (>350ppm). They are further classified as A2-type granites. The whole-rock isotopic compositions of K-feldspar granites (initial 87Sr/86Sr=0.705256–0.706181; εNd(t)=−5.4 to −4.8) and zircon εHf(t) values (−7.8 to 2.0) suggest a mixed magma source. The low zircon Ce4+/Ce3+ ratios (12–88) of K-feldspar granites suggest low oxygen fugacities, which is key for enrichment of tin in primary magmas. The K-feldspar granites have experienced strong differentiation as indicated by their high Rb/Sr and K/Rb ratios, and low Nb/Ta and Zr/Hf ratios, which play an important role in ore-forming element transportation and concentration. A-type granite characteristics of the Xishan pluton show that it formed in an extensional environment. The high F and low Cl characteristics of the K-feldspar granite are most probably attributed to slab rollback. In the Late Cretaceous, the Xishan Sn–W deposit was located near the interaction of the circum-Pacific and the Tethys tectonic realms. Late Cretaceous Sn–W deposits, including the Xishan deposit, form an EW-trending belt from Guangdong to Yunnan Province in South China. This belt is in accordance with the direction of the Neo-Tethys slab rollback in the Late Cretaceous. In addition, the NS-trending extension has been recognized in the Late Cretaceous in South China. We propose that the Xishan Sn–W deposit should be attributed to the Neo-Tethys slab rollback in the Late Cretaceous.

Geophysical and Geochemical Signature of Rare Metal Granites, Central Eastern Desert, Egypt: Implications for Tectonic Environment

Magnetic survey is usually used to delineate magnetic-structural lineaments, analyze their relationships to the inherited ductile fabrics and estimate the depth of perturbing body sources, probably granitic intrusions. These were conducted on the total magnetic intensity reduced to the pole map as well as First vertical derivative and Euler deconvolution maps to show various aeromagnetic structural lineaments which were interpreted as fault systems on the interpretation maps. The early deformational event (D1) produced sets of NE-SW striking local and regional fractures and faults. The second deformational event (D2) generated mainly NNW-SSE and NW-SE faults and fractures some of which intersected earlier (D1) structures. At the northern and eastern parts of the study area (D1/D2) intersections are observed. The last event (D3)created NNE-SSW set of fractures and faults brought out by splay of dykes and reactivated some (D1 and D2)fractures and faults. The study area is also characterized by a major, N-S trending, late-stage dyke system that extend through the area. In order to estimate source depths from gridded aeromagnetic data, 3-D Euler deconvolution method was applied. The calculated source depths are in the range of 200 m to 3500 m. The deepest structures are in the ENE-WSW direction and have depths ranging from about 1100 m to 3000 m in the southeastern part of the study area. On the other hand, the network of parallel major structures trending in NNW-SSE direction have a shallow depth of about 700 m. Rare-metal granites of the Central Eastern Desert of Egypt are classified chemically into alkaline to peralkaline and peraluminous granites. They display the typical geochemical characteristics of A-type granites, with high SiO₂, Na₂O+K₂O, Rb, Zr, Nb, Ta, Sn, and Y, and low CaO, MgO, Ba and Sr. The magmatism of the rare metal granites of the Central Eastern Desert are related to anorogenic, within-plate, A-type, subvolcanic setting and emplaced in the extensional tectonic regime along to the inherited ductile fabrics.

The mineralization age of the Makeng Fe deposit, South China: implications from U–Pb and Sm–Nd geochronology

The Makeng Fe deposit is located in the southwestern Fujian district, South China. The Sm–Nd isochron ages of seven samples of pure garnet and five of pure magnetite separates from the Makeng ores yielded an isochron age of 157 ± 15 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the nearby exposed the Dayang–Juzhou (DJ) porphyritic biotite granite and fine-grained syenogranite yielded 206Pb/238U ages of 140.2 ± 1.1 and 140.1 ± 1.0 Ma, respectively. These results suggest that the intrusion of the DJ granite and the Makeng skarn alterations and Fe mineralization are contemporaneous. The DJ granite exhibits geochemical characteristics of A-type granites, including high values of Na2O + K2O (8.13–8.92 wt%), FeOt/MgO (3.4–21.5), and Ga/Al (2.64–3.45 × 10−4), and low Al2O3 (10.71–13.29 wt%) value. Chondrite-normalized rare earth element patterns are characterized by obviously negative Eu anomalies (δEu = 0.02–0.28) and primitive-mantle normalized spidergrams show the enrichment in high field strength element and depleting in Sr, Ti, Ba, and Eu. The geochemical characteristics of DJ granite suggest that the granite was derived from partial melting of the Paleoproterozoic metasedimentary rocks of the Cathaysia basement. And some underplating of mafic magma in the lower tholeiitic crust and/or depleted mantle might be involved and provide the heat source for the partial melting. The DJ granite also fits the spatiotemporal distribution of the Jurassic–Cretaceous coastward migration of both extensional and arc-related magmatism and fills the A-type granites gap in the early stage of the early Cretaceous (145–125 Ma). Therefore, it is suggested that the late Jurassic and early Cretaceous magmatism in southwestern Fujian district were generated in an extensional environment responding to the slab rollback and concomitant retreating arc system of the paleo-Pacific plate within the South China Block. And the Fe metallogeny in southwestern Fujian district is genetically linked with the magmatism during this period.

Zircon U–Pb geochronology, geochemistry and tectonic implications of Triassic A-type granites from southeastern Zhejiang, South China

The late Permian–Triassic granites in southeastern China have important tectonic significance for the evolution of East Asia. A detailed study utilizing zircon U–Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry for Dashuang and Jingju granites in Zhejiang Province, South China was performed. LA-ICP-MS zircon U–Pb analyses yielded ages of quartz-monzonite and monzogranite that are 234±3Ma and 231±3Ma, respectively, from the Dashuang pluton, while the LA-ICP-MS zircon U–Pb ages of monzogranite and K-feldspar granite from the Jingju pluton are 246±2Ma and 241±3Ma, respectively. These ages indicate that the magmatism event took place during the early Triassic to Mid-Triassic. The two granites have high contents of total alkalis (Na2O+K2O=7.73–10.24%), high-field-strength elements (e.g., Zr=215–471ppm, Y=25.8–36.5ppm, Nb=15–28ppm, and Zr+Nb+Ce+Y=293–849ppm) and rare earth elements (total REE=299–701ppm), as well as high Ga/Al ratios (10,000×Ga/Al=2.44–2.9). The zircon saturation temperatures were 800–837°C for the two granites, which suggests that they have the petrographic and geochemical characteristics of A-type granites. In-situ Hf isotopic analyses revealed that the two granites have εHf(t) values ranging from −20 to −6 and two-stage depleted mantle Hf model ages from 1.6Ga to 2.6Ga, which indicate that the two granite magmas were formed by the partial melting of Paleoproterozoic crust rocks in the Cathaysia Block. A series of A-type granites distributed in the coastal region probably defines an extensional environment in the early Triassic.

Geochronology, elemental and Nd–Hf isotopic geochemistry of Devonian A-type granites in central Jiangxi, South China: Constraints on petrogenesis and post-collisional extension of the Wuyi–Yunkai orogeny

Abstract Details on processes of the early-middle Paleozoic Wuyi–Yunkai orogeny in South China remain poorly defined. Most Silurian–Devonian granites in South China are S-type or I-type granites, which are suggested to be petrogenetically related to the Wuyi–Yunkai orogeny. This paper firstly reported a systematic study on two Devonian A-type granites in the central Jiangxi Province. LA-ICP-MS zircon U–Pb dating results imply that the Huitong and Epo granites were emplaced at about 415 Ma. Both of the two granites have the petrographic and geochemical characteristics of A-type granites. Interstitial biotites occur along the boundary of euhedral plagioclase and quartz and they formed later than plagioclase and quartz. It implies that the primary magma could have been anhydrous. Biotites from the two granites are Fe-rich and have high Fe 2 + /(Fe 2 + + Mg 2 + ) ratios (0.60–0.74). The magmatic temperatures estimated from zircon saturation thermometer are 802–920 °C for the two granites, higher than common I-type and S-type granites. The two granites show high contents of total alkalis (Na 2 O + K 2 O = 6.96–9.39 wt.%), high field strength elements (e.g. Zr = 181–437 ppm, Y = 22.1–39.7 ppm, Nb = 18.6–30.3 ppm and Zr + Nb + Ce + Y = 324–555 ppm), rare earth elements (total REE = 155–312 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 2.50–3.44). These geochemical characteristics are similar to those of A-type granites. The Huitong and Epo granites have relatively low e Nd (t) values of − 10.4 to − 7.7, and low zircon e Hf (t) values (peak value of − 8.0). Whole-rock two stage Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.78 Ga to 2.00 Ga. According to these data, we suggest that the two granites might have derived from partial melting of pre-Cambrian sedimentary rocks which had been granulitized during an earlier thermal event. The two granites contain abundant contemporaneous mafic microgranular enclaves (MMEs). The MMEs display igneous textures and contain acicular apatites, suggesting quenching of mafic magmas that have co-mingled with the host granites. The MMEs might have derived from the enriched lithospheric mantle source. This study on the Devonian Huitong and Epo A-type granites, together with previous studies on Paleozoic granites in South China, indicated that the Wuyi–Yunkai orogeny in South China had changed from syn-collisional crustal thickening to post-collisional extension at least from 415 Ma.

Zircon U–Pb chronology and elemental and Sr–Nd–Hf isotope geochemistry of two Triassic A-type granites in South China: Implication for petrogenesis and Indosinian transtensional tectonism

A detailed study utilizing zircon U–Pb dating, major and trace element geochemistry, and Sr–Nd–Hf isotope geochemistry has been carried out for the Caijiang granite in Jiangxi Province and the Gaoxi granite in Fujian Province, South China. The new data indicate that the Caijiang and Gaoxi granites are Triassic (228–230Ma) and have the petrographic and geochemical characteristics of A-type granites. In both granites, biotite occurs along the boundary of euhedral plagioclase and quartz, which implies that the primary magma could have been anhydrous. The two granites show high contents of total alkalis (Na2O+K2O=7.81–12.15%), high field strength elements (e.g. Zr=240–458ppm, Y=16.8–38.0ppm, Nb=13.5–33.8ppm and Zr+Nb+Ce+Y=382–604ppm) and rare earth elements (total REE=211–373ppm) as well as high Ga/Al ratios (10000×Ga/Al=2.41–3.53). The lowest magmatic temperatures estimated from zircon saturation thermometer were 800–840°C for the Caijiang granite and 820–850°C for the Gaoxi granite, respectively. The Caijiang granite has relatively high (87Sr/86Sr)i ratios of 0.71288 to 0.72009, low εNd(t) values of −9.9 to −9.3, and low zircon εHf(t) values (peak value of −7.5). Whole-rock Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.65Ga to 1.80Ga. The Gaoxi granite has also high (87Sr/86Sr)i ratios of 0.71252 to 0.71356, low εNd(t) value of −13.8 and low zircon εHf(t) values (peak value of −12.0). Whole-rock Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.95Ga to 2.10Ga. According to these data, we suggest that the two granites might have been derived from partial melting of Precambrian crustal rocks that had been granulitized during an earlier thermal event. Our study of the Caijiang and Gaoxi granites, together with previous studies on two Triassic alkaline syenites (Tieshan and Yangfang) in Fujian Province and one A-type granite (Wengshan) in Zhejiang Province in South China, indicate a wide transtensional tectonic environment in the Cathaysia Block that lasted at least from 254Ma to 225Ma. Combined with extant data for the Indosinian granites and tectonic evolution in South China, we suggest that the formation of A-type granites was related to the local NE-trending extensional faults probably caused by collision between the South China Block and the Indochina Block or the North China Block.

Petrological and Geochemical Constraints in the Origin and Associated Mineralization of A-Type Granite Suite of the Dhiran Area, Northwestern Peninsular India

Neoproterzoic, anorogenic, A-type granites of Dhiran area of Malani Igneous Suites is made up wholly of peralkaline-peraluminous alkali feldspar granites and is composed of K-feldspar, quartz, alkali amphibole, plagioclase, biotite and accessory minerals are iron oxides, monozite, zircon, apatite, and annite. This granitoid is associated with acid volcanic and minor basic volcanic-plutonic and dykes. Petrographically, they show cloudy, patchy perthitic texture and graphic texture. They are highly evolved ferroan, alkaline, A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO2, Na2O + K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce and rare earth elements (REE) and low CaO, MgO, Ba and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal origin. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. These granites are high to medium content of radioactive element (U, Th, K) together Heat generation and Heat Production (HP). Leucocratic granites show higher Heat Production (4.84-8.75 µWm-3) and total Heat Generation Unit (11.52-20.84 HGU) than pink granite (2.09-2.94 µWm-3 (HP), 4.98-7.07 HGU). The granites of the Dhiran area show higher average value of total Heat Generation Unit (16.51 HGU and 5.79 HGU) than the average value of 3.8 HGU for the continental crust. The high Heat Generation Unit values (4.98-20.84) of the Dhiran granites indicate ‘hot crust’ category and a possible linear relationship between the surface heat flow and crustal heat generation in the Malani Igneous Suite.

Geochemistry and petrogenesis of Neoproterozoic A-type granites at Nakora in the Malani Igneous Suite, Western Rajasthan, India

The Nakora Ring Complex (NRC) (732 Ma) occurs as a part of Malani Igneous Suite (MIS) in the Western Rajasthan, India. This complex consists of three phases (volcanic, plutonic and dyke). Geochemically, the Nakora granites are peralkaline, metaluminous and slightly peraluminous. They display geochemical characteristics of A-type granites and distinct variation trends with increasing silica content. The peralkaline granites show higher concentrations of SiO2, total alkalies, TiO2, MgO, Ni, Rb, Sr, Y, Zr, Th, U, La, Ce, Nd, Eu and Yb and lower concentrations of Al2O3, total iron, Cu and Zn than metaluminous granites. AI content is ≥1 for peralkaline granites and <1 for peraluminous and metaluminous granites. Nakora peralkaline granites are plotted between 4 to 7 kb in pressure and are emplaced at greater depths (16–28 km and 480–840°C) as compared to metaluminous granites which indicate the high fluorine content in peralkaline granites. The primitive mantle normalized multi-element profiles suggest that Nakora granites (peralkaline, metaluminous and peraluminous) are characterized by low La, Sr and Eu and relatively less minima of Ba, Nb and Ti which suggests the aspects related to crustal origin for Nakora magma. The Nakora granites are characterized as A-type granites (Whalen et al., 1987) and correspond to the field of “Within Plate Granite” (Pearce et al., 1984). Geochemical, field and petrological data suggest that Nakora granites are the product of partial melting of rocks similar to Banded Gneiss from Kolar Schist Belt of India.

Zircon REE patterns and geochemical characteristics of Paleoproterozoic anatectic granite in the northern Tarim Craton, NW China: Implications for the reconstruction of the Columbia supercontinent

Because Archean basement rocks are sparsely distributed around the Tarim Basin, little is known of the relationship between the Tarim Craton and the Paleo- to Mesoproterozoic Columbia supercontinent. Zircon U–Pb dating of a Paleoproterozoic gneissic granite in the northern Tarim Craton yielded a crystallization age of 1915±13Ma, consistent with global Paleoproterozoic collisional events (ca. 2.1–1.8Ga) recorded in most cratons. Despite the fact that some zircons display discordant U–Pb ages with a distinct loss of radiogenetic Pb, all grains have similar 176Lu/177Hf and 176Hf/177Hf(t) values. Zircons from this granite have high Th/U ratios (0.15–0.95), but contrasting rare earth element (REE) patterns. The REE patterns show that the incorporation of most REE elements correlates with increasing age discordance. The discordant zircons, unlike typical magmatic zircons, were probably formed in an open system with the participation of aqueous fluids or hydrous melts during late- to post-magmatic recrystallization. Rocks of this granite pluton have high contents of SiO2 (75.45–78.49wt.%) and total alkali (6.38–9.00wt.%), and moderate Al2O3 (10.31–13.80wt.%), showing peraluminous characteristics. Their high contents of high field strength elements (HFSE) and significant negative Eu anomalies are consistent with the geochemical characteristics of A-type granites. The low ɛHf(t) values of the granite suggest a derivation through anatexis of late Neoarchean tonalite–trondhjemite–granodiorite (TTG) rocks of the basement, indicating that the geochemical characteristics of A-type granites may originate from partial melting of Archean TTGs. In combination with previously published age data, the regional anatexis provides evidence of a late Paleoproterozoic orogenic event on the northern margin of the Tarim Craton. Because the Paleoproterozoic and Mesoproterozoic sedimentary rocks in the northern Tarim Craton show characteristics of sediments deposited on a passive continental margin, the craton is suggested to have occupy an outboard position in the Columbia supercontinent.

THE MESOPROTEROZOIC MUCAJAI ANORTHOSITE - MANGERITE - RAPAKIVI GRANITE COMPLEX, AMAZONIAN CRATON, BRAZIL

The Mesoproterozoic Mucajai anorthosite – mangerite – rapakivi granite (AMG) complex, in northern Brazil, comprises rapakivi granites, fayalite–pyroxene quartz mangerite and syenite, and a massif-type anorthosite forming an asymmetrically zoned igneous complex without counterpart in the Amazonian craton. The rapakivi granites consist of biotite–hornblende granite, including pyterlites and subordinate wiborgites, and porphyritic biotite granite, and exhibit strong similarities with the classic rapakivi granites of Finland. The presence of a fayalite + ilmenite + quartz assemblage in the mangerites indicates low f (O 2 ) conditions during crystallization. The mangerites and rapakivi granites exhibit geochemical characteristics of A-type granites, including high contents of HFS elements (especially Zr), high Ga/Al, Rb/Ba and Rb/Sr values, high K 2 O contents and high FeO t /(FeO t + MgO), typical of reduced rapakivi granites. The biotite granite and the biotite–hornblende granite are related to each other by fractional crystallization, but the mangeritic rocks probably have an independent origin. A 1538 ± 5 Ma Pb–Pb evaporation age has been obtained for a fayalite–pyroxene mangerite. This age overlaps within analytical errors with the 1527 ± 7 Ma (U–Pb SHRIMP) and 1544 ± 42 Ma (U–Pb conventional method) ages previously reported for the anorthosite and rapakivi granites, respectively. The Nd data, including T DM model ages, in the range of 2.07–2.01 Ga, and e Nd (t) values between −2.4 and −1.3 suggest a juvenile crustal source for the granitic rocks of the complex. The e Nd (t) value calculated for the anorthosite (−2.9) probably reflects crustal contamination. Prominent bodies of rapakivi granite coeval with the Mucajai AMG complex are distributed along a NW–SE belt in the western part of the Guyana shield. The Mucajai AMG complex intruded 1.94 Ga country rocks at least ca . 400 my after the stabilization of this part of the Guyana shield, indicating an intraplate setting, in a geological scenario similar to those of the classical rapakivi complexes of the Fennoscandian shield.

Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO2, Na2O + K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.

Petrology and geochemistry of paleoproterozoic A-type granite at Kanigiri in the Nellore–Khammam schist belt, Andhra Pradesh, India

The Kanigiri Pluton (6.2 × 2.1 km 2; 995 Ma) occurs close to a major fault and is intrusive into the meta-volcanics in the western margin of the Nellore–Khammam schist belt in Andhra Pradesh, India. The dominantly metaluminous pluton displays geochemical characteristics of A-type granites, including high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu, displaying distinct variation trends with increasing silica content. It has less fractionated REE patterns and large negative Eu anomalies. The cluster analyses of major and trace elements indicate three main associations: (a) A primary magmatic association characterized by the presence of La, Ce, Fe, Ti, Mn, K, Ba, and Sr, (b) Ore related association consisting of Zr, Hf, Nb, Y, and (c) a volatile association with Rb, Ta, Li, and Cs. Systematic variation in Sr, Rb/Sr, Rb/Ba ratios along with and low K/Rb ratios, reflects fractionation of feldspars. Variations in Sc, and Ta result from fractionation of ferromagnesian silicates. Removal of zircon and allanite affected the Hf/Ta and Ce/Yb ratios. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with the results of melting experiments and indicates that metaluminous granitoids of this pluton were probably derived through melting of lower crustal sources. Geochemical, field and petrological data for the Kanigiri granite suggest that the granite is a partial melting product of mafic rocks like gabbros that are also present in the study region.

Shrimp U–Pb zircon geochronology, geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China

The Cida A-type granitic stock (∼ 4 km 2) and Ailanghe I-type granite batholith (∼ 100 km 2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO ⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites. The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios ( ε Nd( t) = − 0.25 to + 1.35 and ( 87Sr/ 86Sr) i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε Nd( t) ratios (− 6.34 to − 6.26) and high ( 87Sr/ 86Sr) i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.

Neoproterozoic A-type granitoids of the central and southern Appalachians: intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765–680 and 620–550 Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeOt/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U–Pb zircon ages obtained from the Polly Wright Cove (706±4 Ma) and Suck Mountain (680±4 Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U–Pb zircon ages for the Pound Ridge Granite Gneiss (562±5 Ma) and Yonkers Gneiss (563±2 Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous granitoids of the supersuite likely were derived through melting of lower crustal sources. Mildly peralkaline rocks of the Robertson River batholith and Irish Creek pluton may be derived from more chemically primitive sources similar in composition to ocean–island basalts. Blue Ridge granitoids define a plutonic episode that occurred during an unsuccessful pulse of crustal extension which predated opening of Iapetus by more than 100 million years. Granitoid gneisses in New York were emplaced during an extension-related, dominantly mafic magmatic episode that ultimately led to development of Iapetus.