Granitoid Bodies Research Articles

U–Pb zircon chronology and petrogenesis of Carboniferous plutons in the northern part of the Eastern Pontides, NE Turkey: Constraints for Paleozoic magmatism and geodynamic evolution

Numerous intrusive rocks of varying ages and compositions exist in the Paleozoic to Tertiary periods in the Eastern Pontides. Carboniferous intrusive rocks are commonly observed in the southern part of the Eastern Pontides. The nature of the rocks in the northern part of the region has not been determined because of Upper Cretaceous and Tertiary volcano-sedimentary units. Whole-rock geochemical, isotopic and geochronological data obtained from five different mapped granitoid bodies located in the northern part of the Eastern Pontides allow for the proper reconstruction of Carboniferous magmatism and the geodynamic evolution of the region.According to laser ablation ICP-MS U–Pb zircon dating, the Özdil, Soğuksu, Seslikaya, Kızılağaç and Şahmetlik plutons have similar 206Pb/238U vs. 207Pb/235U concordia ages of 340.7±1.8Ma and 323.1±1.5Ma, 348.4±1.6Ma, 335.4±1.4Ma, 337.2±0.6Ma and 334.5±1.4Ma, respectively. The aluminium saturation index (ASI) values of all of the samples from the plutons are between 1.0 and 1.32, which indicate peraluminous melt compositions. The plutons have SiO2 contents between 59 and 79wt.% and show low- to high-K calc-alkaline characteristics. The plutons are enriched in large-ion lithophile and light rare earth elements and are depleted in high-field strength elements. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies, with LaCN/YbCN=1.9–46.8 and EuCN/Eu*=0.19–1.76. The studied plutons show considerable variations in 87Sr/86Sr(i) (0.70255 to 0.71006) and εNd(i) values (−4.8 to −7.1), as well as Nd model ages (1.15 to 2.47Ga). The Pb-isotopic ratios are 206Pb/204Pb=17.11–18.60, 207Pb/204Pb=15.58–15.64 and 208Pb/204Pb=36.95–38.62. The crystallization temperatures of the melts range from 676 to 993°C, as determined by zircon and apatite saturation thermometry.These data suggest that the Carboniferous granitic magmas were produced by the partial melting of meta-mafic to meta-felsic lower crustal source rocks, with minor contributions from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the northern part of the Eastern Pontides.

The temporal evolution of the active margin along the Southeast Anatolian Orogenic Belt (SE Turkey): Evidence from U–Pb, Ar–Ar and fission track chronology

The Southeast Anatolian Orogenic Belt (SAOB) resulted from the north-dipping subduction of the southern Neotethyan oceanic lithosphere in late Mesozoic and early Cenozoic. However, the timing and the rate of the continental collision are still under debate. Here, we present new U–Pb, Ar–Ar and Fission Track ages from the I-type calc-alkaline granitoids (Esence, Doğanşehir and Baskil) cutting the Göksun, İspendere and Kömürhan ophiolites and also the Malatya–Keban metamorphics representing the Tauride active continental margin along the SAOB. In this study, high- to low-temperature thermochronological methods, applied to the granitoids, are used to understand the temporal and geodynamic evolution of the Tauride active continental margin and continental collision between Tauride and Arabian platforms in the frame of the Neotethyan convergence during Cretaceous and Miocene. The U–Pb zircon ages range from 83 to 88Ma for the Baskil and from 81 to 83Ma for the Esence granitoids. The 39Ar–40Ar ages indicate that these late Cretaceous granitoids cooled below 300°C in 6–10Ma. The formation ages and the timing of the gradual cooling of the late Cretaceous granitoids are similar to metamorphism age and the timing of the exhumation of the HP/LT Bitlis metamorphics during this stage. The combined field, geochemistry and geo-thermochronological data suggest that the first continental collision event occurred between Bitlis–Pütürge micro-continents to the south and Tauride platform to the north in an oblique subduction zone between 84 and 74Ma. The granitoids continued uplifting during early-middle Eocene together with the exhumation of HP/UHT Berit metaophiolite in an extensional regime related to the opening of the Maden back-arc basin. This period also let the intrusion of the Doğanşehir arc magmatism, intruding the Pütürge and Malatya–Keban metamorphics, Berit metaophiolite, and Maden Complex. The Eocene Doğanşehir granitoid has similar U–Pb and 39Ar–40Ar ages, suggesting a fast cooling. The apatite fission track (AFT) ages for all granitoid bodies suggest that they were mainly cooled or uplifted in two episodes, where the first one is in the Eocene in an extensional setting. The AFT data marked the final continental collision between the Taurides and the Arabian platform first in Oligocene and the break-off of the subducted slab and the delamination process caused fast uplift of the Eastern Anatolia during middle to late Miocene.

Geodynamic evolution of a Pan-African granitoid of extended Dizo Valley in Karbi Hills, NE India: Evidence from Geochemistry and Isotope Geology

The Dizo Valley is an important geological province in NW Karbi Hills of Shillong Plateau (NE India). The Karbi Hills contain profusely intruded bodies of A-type granitoids that have been attributed to a post-collision setting. The Dizo Valley magmatic suite is a bimodal association of voluminous granitoid plutons with dolerite and amphibolites (metabasalts). Our present data demonstrates that there were two episodes of A-type granitoid magmatism in this part of the craton. Studied Kathalguri granitoids (KG) of Dizo Valley represents the late phase, about 515.1±3.3–515.5+2.7Ma age, dated by zircon LA-ICPMS U–Pb method. The age of emplacement represent the late Pan-African (Mid-Cambrian) event of granitoid magmatism concomitant to the Gondwana continental restructuring event with the integration of East and West Gondwana held between 570 and 510Ma. A previous granitoid emplacement episode of about 690Ma has been referred in literature. The late phase of granitoids formation tends to be massive, occasionally show flow banding, degassing pots and amphibolite breccias with chilled margin. Its mineralogy shows abundance of accessory phases like zircon, allanite, apatite, bastnaesite, monazite, sphene and xenotime.Geochemical parameters offer metaluminous to peraluminous, non-porphyritic, shoshonitic to high potassium, calc-alkaline composition. Other chemical criteria bear the character of A2-type granitoids with high SiO2 (64.00–70.75wt.%) and alkali (3.26–6.30wt.%) but poor Ca and Mg content. The binary plots of Y+Nb–Rb and Y–Nb confirm their within-plate granite (WPG) character. The pluton is enriched in total REE (av. 614.64ppm), Y and heavy REE (27.62–65.00ppm; av. 41.87ppm) compared to low calcium granite; Eu anomaly is moderately negative (∂Eu=0.43). Enrichment of incompatible elements like large ion lithophile elements (LILE) of Rb, Ba and Sr and REE are consistent with the A-type granites. The behavior of Ba, Rb and Sr suggest a progressive enrichment of Ba for the studied granitoids and a gradual depletion of Rb, Sr and Eu values. The Lu–Hf profile produced an initial 176Hf/177Hf (i) ratio of all zircons from 0.2819–0.2821 and have ɛHf(t) values −12.20 to −18.90; suggesting melting of 2.25–2.67Ga old metasomatized crust (TDM2) of magmatic origin which has been predicted to produce KG granitoids that had gained enrichment in LILE during earlier subduction events. The zircons bear evidence of oscillatory zoning visible in Cathodo Luminescence (CL) images which clearly attributes toward a magmatic origin of KG granitoids.

Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints

The Natal Metamorphic Province consists of, from north to south, the Tugela, Mzumbe, and Margate Terranes. These were accreted to the southeastern margin of the Kaapvaal Craton in the late Mesoproterozoic, and followed by intrusion of a large suite of A-type granitoid bodies. New U–Pb data from zircon, titanite, and monazite further constrains the temporal framework of these geological events.The Tugela and Mzumbe Terranes record protracted magmatism in an island arc complex from ∼1200Ma to 1160Ma, followed by the accretion of these terranes onto the southern margin of the Kaapvaal Craton at ∼1150Ma. Arc magmatism in the Margate Terrane continued until ∼1120Ma and was followed by extension and bimodal volcanism immediately prior to accretion to the Kaapvaal/Mzumbe continental margin at ∼1090Ma. This accretion was accompanied by high-pressure and high-temperature metamorphism, juxtaposition of the Mzumbe and Margate Terranes along the Melville Thrust, and the formation of a number of syntectonic intrusive units derived from melting of the pre-existing arc crust. After accretion, extensional collapse is evidenced by the intrusion of mafic/ultramafic and alkaline intermediate magmatic suites at ∼1085Ma, resulting from mafic underplating and/or lower crustal delamination. Nd and Hf isotopic data imply the magmatic rocks of the Natal Metamorphic Province were derived from relatively juvenile continental crust, initially generated by island arc magmatism and subsequently reworked during the accretion event(s). The combined Kaapvaal-NMP region (the southern margin of the enlarged Kalahari Craton) then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the Kaapvaal cratonic margin to the southernmost portion of the Natal Metamorphic Province. This deformation is accompanied by low-pressure, (ultra)high-temperature metamorphism, isobaric cooling, and intrusion of the voluminous A-type Oribi Gorge Suite porphyritic granites and charnockites throughout the Mzumbe and Margate Terranes.

Zircon U–Pb age and Hf isotopic compositions of Mesozoic granitoids in southern Qiangtang, Tibet: Implications for the subduction of the Bangong–Nujiang Tethyan Ocean

The southern Qiangtang magmatic belt was formed by the north-dipping subduction of the Bangong–Nujiang Tethyan Ocean during Mesozoic. To better understand the petrogenesis, time–space distribution along the length of this belt, 21 samples of several granitoid bodies, from west to east, in the Bangong Co, Gaize, Dongqiao and Amdo areas were selected for in-situ zircon U–Pb dating, Hf isotopic and whole-rock chemical analyses. The results suggest a prolonged period of magmatic activity (185–84Ma) with two major stages during the Jurassic (185–150Ma) and the Early Cretaceous (126–100Ma). Both the Jurassic and Cretaceous granitoids are high-K calc-alkaline I-type rocks, except the Cretaceous two-mica granite from Amdo in the east, which belongs to S-type. The granitoids are generated from different source materials as indicated by zircon Hf isotopic compositions. The Bangong Co and Dongqiao granitoids show high zircon εHf(t) values of −1.3–13.6 with younger TDMC ages of 293–1263Ma, suggesting a relatively juvenile source; whereas the Gaize and Amdo granitoids have low εHf(t) values of −16.1–2.9 with older TDMC ages of 999–2024Ma, indicating an old crustal contribution. These source rocks melt at different P–T conditions as suggested by Sr/Y ratio and TZr. The Sr/Y ratio of both stage granitoids increases with decreasing age. However, the TZr of the Jurassic granitoids decreases, whereas the TZr of the Cretaceous granitoids increases with decreasing age. The contrasting geochemical signatures of these granitoids may be controlled by the varying contribution of slab-derived fluids involved in the generation of the Jurassic and Cretaceous granitic magmas; i.e. increasing amount of fluids in the Jurassic, whereas decreasing amount of fluids in the Cretaceous. Therefore, it is proposed that the Jurassic and Cretaceous magmatism may be related to subduction and closure of the Bangong–Nujiang Tethyan Ocean, respectively. The age pattern of the Jurassic and Cretaceous granitoids suggests an oblique subduction of the Bangong–Nujiang Tethyan Ocean and a diachronous collision between the Lhasa and Qiangtang blocks.

Geochemistry and geochronology of granitoids in the Kibi-Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana

In Ghana the West African Craton is represented by Birimian and Tarkwaian rocks with extensive granitoid bodies. Granitoids from Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana were analysed for major and trace element contents and found to be characterised by highly-fractionated REE, enrichments, in LILE, and depletion in Nb, Ta and Sr. The LILE enrichment relative to strong Nb–Ta depression, indicates that these granitoids were emplaced in an active margin. Based on field relations, geochemical composition and geochronological data, the granitoids from the Kibi-Asamankese area can be divided into three types, namely; the Eburnean biotite granodiorite (2133–2127Ma) and hornblende granodiorite (2147Ma), and the Pre-Eburnean gneissic biotite granite (2193Ma). The geochemical data of the studied rocks plot in the tholeiitic field, whereas on the A/CNK–A/NK diagram, they generally fall within the metaluminous field, with A/CNK values between 0.69 and 0.88. U–Pb dating of zircons in the granitoids yielded ages ranging from 2193 to 2127Ma, which are among the oldest ages obtained from the granitoid plutons in Ghana. Such high-precision geochronological data indicate that magmatism occurred over a time-span of about 70Ma. This provides further evidence that the period 2.1–2.2Ga was one of the important stages of Birimian magmatism that led to the generation of the granitoids. From the above-mentioned ages, it is possible to link the geological activities to crustal processes and establish the cyclic geotectonic evolution in the West African Craton over time as part of an arc-back-arc basin system.

Petrogenesis and implications for tectonic setting of Cambrian suprasubduction-zone ophiolitic rocks in the central Beishan orogenic collage, Northwest China

The Hongliuhe ophiolite is a fragment of the early Paleozoic oceanic crust situated in a suture zone central to the Beishan orogenic collage, which is a subset to the Central Asian Orogenic Belt in Northwest China. It has lithologic, geochemical and chronological similarities to the Yueyashan–Xichangjing ophiolite 400km east, along strike, allowing for correlative interpretations of marginal seafloor spreading processes. The Hongliuhe ophiolite is explored in this study with complete petrologic, geochemical and structural descriptions. The ultramafic body comprises cumulate-layered lherzolite and gabbro plutonic lithologies, tectonized with syn-magmatic extension. A U–Pb age of 520.3±5.8Ma was obtained by SHRIMP analysis of zircons from a pegmatitic gabbro. The ultramafic rocks are variably depleted and some show refertilization of LILE’s consistent with slab dehydration in a supra-subduction mantle wedge. Mineral chemistry shows high Cr# (36–82) populations of spinel, and wide-ranging lower Mg# (8–50), consistent with known forearc peridotite chemistry partially re-equilibrated by tectonism or through staged magmatism. Olivine and pyroxene chemistries are consistent with this interpretation. A thin volcanic cover is interlayered with red cherts, and has a bimodal set of geochemical signatures, both island-arc tholeiite and calc-alkaline. They bear distinct Nb–Ta anomalies consistent with slab-derived fluid initiated melting in a supra-subduction environment. Coarse volcaniclastic turbidites infilled paleo-topography along the ophiolite, and they are interbedded with quartzofeldspathic sandstones, indicating an initial proximity to an arc. The sandstones contain detrital spinel with analyses showing relation to both the ophiolite and possibly a secondary alkalic volcanic source. The supracrustal sequence continues with a thick, contiguous covering of margin-related rocks, including interlayered limestone, pyroclastic and siliciclastic rocks, and basalts. As a constraint on emplacement we provide an U–Pb Zircon SHRIMP date (413.6±3.5Ma) from an undeformed granitoid body intruded within the overlying stratigraphy. The entire analysis paints a picture of subduction rollback-related extension affecting the margin of an emerging or mature arc just prior to a known magmatic cycle that ended with a collision in the Devonian at the latest. This suture is one of many in the Beishan, and our interpretation provides evidence in a long running debate supporting accretionary orogenesis in an island-arc archipelago as the main system for growing sialic crust in the Paleozoic central Asia.

Constrains on vorticity and non-coaxial shear direction in Neoarchean L-S tectonites, an example from northern Minnesota, USA

We present a detailed kinematic study of seven Neoarchean L-S tectonite samples in order to constrain vorticity and non-coaxial shear direction relative to foliation and elongation lineation. Samples are L-S tectonites from the Wawa Subprovince of the Archean Superior Province, more specifically the Vermilion District of NE Minnesota, a NE-trending belt of greenschist grade supracrustal rocks and granitoid bodies. Supracrustal rocks host multiple L-S tectonite packages with a well-developed sub-vertical metamorphic foliation and elongation lineation; elongation lineation generally plunges steeply to gently, although zones of shallow plunge occur locally. The Wawa Subprovince is widely interpreted as a transpressional plate margin with shear zones recording unidirectional dextral strike-slip, an interpretation held up as fundamental evidence for Archean plate-tectonic processes. However, vorticity and shear direction within Vermilion District L-S tectonites remain unconstrained. We compare data from thin-sections, X-ray computed tomography, and quartz crystallographic fabric analysis to monoclinic shear models to constrain vorticity and better understand the geometric relationships between vorticity, non-coaxial shear direction, foliation, and elongation lineation. Kinematic indicators in thin-section and image slices from X-ray computed tomography consistently record asymmetric microstructural fabrics in foliation-normal/lineation-parallel planes, whereas planes normal to foliation and elongation lineation display dominantly symmetric microstructural fabrics. Mantled porphyroclast 3D-shapes and star-volume distribution analyses indicate that porphyroclast short-axes are normal to foliation and long-axes parallel elongation lineation. Quartz crystallographic preferred orientation data show a-axes maxima sub-parallel to foliation-normal/lineation-parallel planes. Kinematic data consistently show a vorticity vector within the foliation plane and normal to elongation lineation; thus non-coaxial shear direction is sub-parallel to elongation lineation. Data are inconsistent with shear models in which non-coaxial shear direction is normal to lineation, or in which the vorticity vector is normal to foliation. Rather, kinematic data indicate that tectonites record non-coaxial shear broadly parallel to elongation lineation regardless of the geographic orientation of lineation.

Two-stage formation of the alcaline volcano-plutonic complexes in the Ketkap-Yuna igneous province of the Aldan Shield: New isotopic data

1322 Recently, traditional views on the time limits accepted for the formation of many geological objects have been revised based on dates obtained for monom ineral fractions by high precision U–Pb and 40Ar/39Ar methods. The Ketkap–Yuna igneous province representing a tectonotype of the zones of Mesozoic tectono mag matic activation in the Adan Shield is one such object (Fig. 1; a geological–structural map of the province is available in [9]). The Ketkap–Yuna igneous province is a unique structure characterized by several polyformation mag matic complexes, which are discretely distributed through the latter. Its western segment (Bokur vol cano pluton of the Ulakhan massif) demonstrates wide development of alkaline, volcanic (Bokur Com plex), and subvolcanic (Kurung Complex) rocks. The central and eastern parts of the province are character ized by prevalence of hypabyssal bodies of subalkaline diorites and granitoids, including gold bearing variet ies. Alkaline intrusive rocks (Dar’ya Complex) occur sporadically in form of dike “swarms,” a system of small sills, and small stocks. Alkaline volcano plutonic and intrusive magma tism in zones of tectono magmatic activation is thought to be related to the second stage of Mesozoic magmatism of the Aldan Shield [1, 2, and others]. According to published data, its K–A ages fall into the interval of 168 to 70 Ma (Table 1 and data from [1–8]).

Late Paleozoic metallogenesis and evolution of the East Tianshan Orogenic Belt (NW China, Central Asia Orogenic Belt)

One of the most largest known and important metallogenic provinces in China is East Tianshan, where seven major types of Late Paleozoic metal deposits have been recognized: (1) porphyry-type Cu-Mo-(Au) ore deposits, (2) volcanic Fe-Cu deposits, (3) orogenic lode gold deposits, (4) magmatic Cu-Ni sulfide deposits, (5) epithermal gold deposits, (6) volcanic hydrothermal Cu deposits, and (7) skarn Cu-Ag deposits. Tectonically, the development of these Late Paleozoic metal mineral deposits was closely associated with the subduction and closure of the ancient Tianshan ocean intervening between the Tarim craton and the Junggar-Kazakhstan block. In the Late Devonian to Early Carboniferous, the northern margin of the Tarim craton existed as a passive-type continental margin, whereas the ancient Tianshan ocean was subducted beneath the southern margin of the Junggar-Kazakhstan block, resulting in the formation of the Dananhu-Tousuquan magmatic arc and associated porphyry-type Cu-Mo-(Au) deposits. In the Middle Carboniferous, the ancient Tianshan ocean began to subduct beneath the northern margin of the Tarim craton, leading to the formation of the Aqishan-Yamansu magmatic arc and associated volcanic Fe-Cu deposits. In the Late Carboniferous, the ancient Tianshan ocean was closed, and a continent-arc collision occurred, leading to the formation of the Tianshan orogen. Following the collision was an extensional event, which was associated with the emplacement of large amounts of ultramafic-mafic complexes and the formation of a number of large- to medium-scale magmatic copper-nickel ore deposits along the Kangger suture zone. In the Early Permian, East Tianshan entered into a post-collision stage, associated with the widespread emplacement of granitoid bodies and eruption of within-plate volcanism, which led to the formation of volcanic hydrothermal copper deposits, skarn-type Cu-Ag deposits, post-orogenic gold deposits, and epithermal gold deposits in East Tianshan.

Termination of the Southern Irumide Belt in Tanzania: Zircon U/Pb geochronology

Distinctly different crustal blocks converge in southern Tanzania which are thought to belong to the Paleoproterozoic Usagaran/Ubendian Belt, the Mesoproterozoic Irumide Belt and the Neoproterozoic East African Orogen. Zircons from thirteen samples were dated by LA-SC/MC-ICPMS in order to assign the different crystalline basement rocks to one of the above mentioned geological units based on the age of metamorphism and formation of the igneous precursor rocks. The mainly migmatic gneissic samples were taken along a traverse from Mbamba Bay, Lake Malawi to Masasi, S-Tanzania. Zircon cores and rims trace magmatic protoliths between 1000±9 and 1055±6Ma and metamorphism between 563±10 and 616±12Ma. Ages belonging to the Usagaran/Ubendian event as well as the 640Ma EAO event were only found in the westernmost and easternmost part of the transsect, respectively. Magmatic ages of 724±6 and 764±4Ma were determined in locally confined undeformed granitoid bodies. Mostly similar ages as well as lithological units can be found in the Unango and Marrupa Complexes in N-Mozambique which are considered as part of the Southern Irumide Belt. The exact location of terrane boundary between the Southern Irumide Belt and the Western/Eastern Granulites and the Usagaran/Ubendien Belts in southern Tanzania remains uncertain. With aid of digital elevation data, satellite images and airborne geophysical data we suggest that a combination of fault systems is responsible for the distribution of the different units. The northeastern limit of the Southern Irumide Belt (Unango and Marrupa Complexes) can be traced along a major tectonic boundary, the Mwembesi Fault system, that extends from Malawi to the south of the Mahenge Mountains in Tanzania.

X-ray crystallography and mineral chemistry of bastnaesite from Kanigiri granite, Prakasam district, Andhra Pradesh, India

Abstract The authors report the results of X-ray diffraction (XRD) and geochemical studies on bastnaesites (lanthanum cerium fluoro-carbonate) hosted in alkali Kanigiri Granite of the Prakasam district in Andhra Pradesh, India. The XRD pattern of the investigated bastnaesite displays sharply-defined reflections. The observed d-spacings of the bastnaesite are in very close agreement with those published for bastnaesite standard in International Centre for Diffraction Data (ICDD) Card No. 11–340. The calculated unit cell parameters (a o; c o) and unit cell volume (V) of the studied bastnaesite (a o 7.1301–7.1413 Å, c o 9.7643–9.7902Å and V 429.8940–432.3875 Å3) are almost equal to values published for bastnaesite standard (c o 7.1290 Å, c o 9.7744 Å and V 430.19 Å3) in the relevant data card. Geochemical data of bastnaesite reveals high content of Ce (mean 27.22%) followed by La (mean 16.82%), Nd (mean 6.12%) and Pr (mean 1.91%). Compared to light REE (LREE) content (mean 437165 ppm), heavy REE (HREE) content (mean 5867 ppm) is drastically low, with unusually high LREE/HREE ratio (mean 80). The chondrite-normalised plot also exhibits drastic enrichment of LREE relative to HREE with pronounced negative Euanomaly (mean Eu/Eu* = 0.15). High (LREE)N / (HREE)N, (La/Lu)N, (La/Yb)N and (Ce/Yb)N ratios reveal higher fractionation of LREE relative to HREE. The rare earth element (REE) contents of the studied bastnaesite are very close to REE contents of bastnaesite hosted in alkali syenite from Madagascar. The presence of bastnaesite in Kanigiri Granite and soils derived from it enhances the scope of further exploration for bastnaesite in several bodies of alkaline rocks and alkali granitoids present along the eastern margins of the Cuddapah basin, Andhra Pradesh.

Tracing the Transhimalayan magmatic belt and the Lhasa block southward using zircon U–Pb, Lu–Hf isotopic and geochemical data: Cretaceous – Cenozoic granitoids in the Tengchong block, Yunnan, China

A number of Cretaceous – Cenozoic granitoid bodies in the Tengchong block were analyzed for their U–Pb zircon ages, Lu–Hf zircon isotopic compositions and whole-rock geochemistry. Zircons from six different granite bodies in the central Tengchong block have weighted mean 206Pb/238U ages ranging from 76 to 65Ma, and negative εHf(t) values ranging from −2.7 to −15.5 with TDMC ranging from 1312 to 1928Ma. One dextrally sheared and gneissic granodiorite sample of the Nabang shear zone in the western Tengchong block yielded a weighted mean 206Pb/238U age of 19.7±0.4Ma, and positive εHf(t) values ranging from +0.2 to +10.7 with TDMC ranging from 418 to 1092Ma. The late Cretaceous granites have metaluminous and peraluminous affinities with strong enrichment of U and Th, and significant negative anomalies of Eu, Ba, Sr, Nb, P, and Ti. The Miocene granodiorite displays strong negative anomalies of Nb, Ta, P, and Ti and a weak negative Eu anomaly. The data suggest that the late Cretaceous granites in the Tengchong block were derived from partial melting of ancient, most likely Proterozoic continental crustal material whereas the Miocene gneissic granodiorite was derived from partial melting of juvenile crustal material possibly within a supra-subduction setting and intruded synkinematically into the active Nabang shear zone.Lu–Hf-isotopic data of late Cretaceous – Paleocene granites are indistinguishable from those of granites of the Central Lhasa block. Thus, we tentatively correlate the late Cretaceous granites of the Tengchong block with those of the Central Lhasa block. The boundary between the South and Central Lhasa blocks in western Tengchong may be exposed in the NNE-striking, dextral Nabang shear zone along the Chinese-Myanmar border region, which hosts early Eocene granitoids with εHf(t) values >0. The Lu–Hf-data of the zircons from the gneissic Miocene granodiorite indicate a juvenile crustal substrate in the western Tengchong block as well. The synkinematic Miocene granodiorite provides an important age constraint for dextral shear at mid-crustal levels, which was active until at least c. 19–20Ma in the Nabang shear zone. This Miocene intrusion may be a first indication that active margin tectonics lasted 20–25Ma longer than in the Gangdese belt and possibly was associated with deformation partitioning in the hinterland produced by increasing obliquity of subduction during clockwise rotation of Sundaland during the India–Asia collision.

Geochronology of granitic rocks from the Ruangwa region, southern Tanzania – Links with NE Mozambique and beyond

New U–Pb zircon LA-ICP-MS data are presented for 4 granitoid bodies which intrude high grade gneisses of the previously unmapped Ruangwa region in southern Tanzania. The study area forms part of the late Neoproterozoic East African Orogen (EAO). The oldest unit, a coarse-grained migmatitic granitic orthogneiss gave an early Neoproterozoic (Tonian) crystallization age of 899±9/16Ma, which is similar to, but significantly younger than, Stenian-Tonian basement ages in areas relatively nearby. Crust of this age may extend as far north as the major Phanerozoic Selous Basin, north of which Archaean protolith ages predominate (the “Western Granulites”), except for the juvenile Neoproterozoic “Eastern Granulites”, which are not represented in the study area. To the south, the Tonian crust of the study area provides a tentative link with the Marrupa Complex in NE Mozambique. A granite pluton, dated at 650±5/11Ma is broadly coeval with the main Pan-African tectono-thermal event in the East African Orogen that is recorded across Tanzania north of the Selous Basin. Zircons in this granite contain inherited cores at ca. 770Ma. This age is within the range of dates obtained from south and west of the study area from juvenile granitoid orthogneisses which might be related to a widespread, but poorly understood, early phase of Gondwana assembly along an Andean-type margin.South of the study area, in NE Mozambique, the latest orogenic events occurred at ca. 550Ma, and are sometimes attributed to the Ediacaran-aged “Kuunga Orogeny”. While metamorphic dates of this age have been recorded from the EAO north of the Selous Basin, magmatic rocks of this event have not been recognized in Tanzania. The two youngest granitoids of the present study are thus the first 500–600Ma igneous rocks reported from the region. A weakly deformed very coarse-grained granite pluton was dated at 591±4/10Ma, while a very late, cross-cutting, undeformed granite dyke gave an intrusive age of 549±4/9Ma.The granitoids ages presented in this study contain elements that are characteristic of the northern, Tanzania-Kenya, segment of the East African Orogen and of the southern, Mozambique, segment. The Tonian orthogneiss sample is typical of (but somewhat younger than) the Marrupa Complex of NE Mozambique. No zircon inheritance was recorded in the sample, typical of the juvenile Marrupa Complex. On the other hand, the ca. 650Ma granite pluton has an age that is typical of the northern segment of the orogen; this is the first recorded granite of that age intruded into the Tonian-dominated crust of southern Tanzania or NE Mozambique. The two younger granites have provided dates that are typical of the southern segment of the orogen, and that of the Kuunga Orogen. The study area thus appears to represent an area of transitional crust straddling two complex and contrasting segments of the East African Orogen, with elements of both segments present and evidence for a ca. 770Ma event which appears to be quite widespread and may relate to the early phases of Gondwana amalgamation in southern East Africa.

Thermal evolution of the Tseel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

AbstractThe timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.

Major Types and Characteristics of the Late Paleozoic Ore Deposits in the East Tianshan Orogenic Belt, Central Asia

One of the most largest known and important metallogenic provinces in China is East Tianshan, where seven major types of Late Paleozoic metal deposits have been recognized: (1) porphyry-type Cu-Mo-(Au) ore deposit, (2) volcanic Fe-Cu deposit, (3) orogenic lode gold deposit, (4) magmatic Cu-Ni sulfide deposit, (5) epithermal gold deposit, (6) volcanic hydrothermal Cu deposit, and (7) skarn Cu-Ag deposit. Tectonically, the development of these Late Paleozoic metal mineral deposits was closely associated with the subduction and closure of the ancient Tianshan Ocean intervening between the Tarim craton and the Junggar-Kazakhstan block. In the Late Devonian to Early Carboniferous, the northern margin of the Tarim craton existed as a passive-type continental margin, whereas the ancient Tianshan ocean was subducted beneath the southern margin of the Junggar-Kazakhstan block, resulting in the formation of the Dananhu-Tousuquan magmatic arc and associated porphyry-type Cu-Mo-(Au) deposits. In the Middle Carboniferous, the ancient Tianshan ocean began to subduct beneath the northern margin of the Tarim craton, leading to the formation of the Aqishan-Yamansu magmatic arc and associated volcanic Fe-Cu deposits. In the Late Carboniferous, the ancient Tianshan ocean was closed, and a continent-arc collision occurred, leading to the formation of the Tianshan orogen. Following the collision was an extensional event, which was associated with the emplacement of large amounts of ultramafic-mafic complexes and the formation of a number of large- to medium-scale magmatic copper-nickel ore deposits along the Kangger suture zone. In the Early Permian, East Tianshan entered into a post-collosion stage, associated with the widespread emplacement of granitoid bodies and eruption of within-plate volcanism, which led to the formation of volcanic hydrothermal copper deposits, skarn-type Cu-Ag deposits, post-orogenic gold deposits, and epithermal gold deposits in East Tianshan.

URAN A THORIUM V GRANITICKÝCH PEGMATITECH A APLITECH SILEZIKA

Contents of uranium and thorium were measured using a laboratory gamma-ray spectrometer in 575 samples of granitic pegmatites and aplites from Silesicum. Very low contents of the both elements were found in metapegmatites occurring in metagranitoids and blastomylonites of the Desná Group, in pre-Variscan muscovite pegmatites with tourmaline, and in Variscan beryl-columbite pegmatites. Very high uranium and thorium contents are in some pegmatite dykes occurring on the Kluč Hill near Kociánov. The pegmatites are composed mainly of feldspars, quartz and mica (biotite and/or muscovite), and contain accessory minerals of uranium and thorium such as uraninite, coffi nite and thorite. The Kluč Hill pegmatites are spatially and genetically bound to a small I-type Variscan granitoid body (so-called Rudná hora Intrusion). Highest average uranium contents were found in pyroxenic pegmatites occurring in the Žulová Massif mantle (its marbles can be considered the main source of uranium).

Tectonomagmatic evolution of the Early Ordovician suprasubduction-zone ophiolites of the Trondheim Region, Mid-Norwegian Caledonides

Abstract The Trondheim Region ophiolites in the Mid-Norwegian Caledonides represent variably tectonized ophiolite fragments. We present high-precision thermal-ionization mass spectrometry and secondary-ion mass spectrometry (SIMS) U–Pb zircon dates, whole-rock geochemical and Sm–Nd data and Lu–Hf zircon analyses that permit the timing and nature of various stages in the evolution of the ophiolite to be elucidated. Plagiogranite intrusions dated at 487 and 480 Ma have relatively juvenile Nd and Hf isotopic compositions (ε Nd( t ) =6.3, ε Hf( t ) =8.2–12.4). Geochemical data indicate a subduction-zone influence, suggesting formation in an oceanic back-arc setting. At 481 Ma, a granitoid body with a relatively strong unradiogenic Nd and Hf isotopic composition (ε Nd( t ) =−2.6 to −4.0, ε Hf( t ) =3.8–6.4) and subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect the formation or migration of an oceanic arc above a subduction zone influenced by continentally derived sediments. At c. 475–465 Ma, a greenstone-dominated conglomerate and volcaniclastic sequence was deposited on the eroded ophiolite, indicating obduction between about 480 and 475 Ma. At c. 468–467 Ma, the deformed ophiolite and its sedimentary cover was intruded by trondhjemite dykes and shoshonitic volcanic rocks with intermediate Nd and Hf isotopic compositions (ε Nd( t ) =3.0–3.9, ε Hf( t ) =4.4–10.2). We interpret this magmatism to reflect subduction-polarity reversal and establishment of a magmatic arc at the continental margin shortly after obduction. Supplementary material: Whole-rock geochemistry, Sm–Nd isotopic data, SHRIMP U–Pb zircon, TIMS U–Pb zircon and Lu–Hf isotopic data are available at http://www.geolsoc.org.uk/SUP18689 .

SHRIMP U–Pb zircon geochronology and thermal modeling of multilayer granitoid intrusions: Implications for the building and thermal evolution of the Central System batholith, Iberian Massif, Spain

This work shows the results of a U–Pb SHRIMP zircon geochronological study of the central part of the Gredos massif (Spanish Central System batholith). The studied batholith is composed of several granodiorite and monzogranite tabular bodies, around 1km thick each, intruded into partially molten pelitic metasediments. Granodiorites and monzogranites, belonging to three distinct intrusive bodies, and samples of anatectic leucogranites have been selected for SHRIMP U–Pb zircon geochronology. Distinct age groups, separated by up to 20Ma, have been distinguished in each sample. Important age differences have also been determined among the most representative age groups of the three analyzed granitoid bodies: 312.6±2.8Ma for the Circo de Gredos Bt-granodiorites (floor intrusive layer), 306.9±1.5Ma for the Barbellido-Plataforma granitoids (top intrusive layer) and 303.5±2.8Ma for Las Pozas Crd-monzogranites (middle intrusive layer). These age differences are interpreted in terms of sequential emplacement of the three intrusive bodies, contemporary with the Late Paleozoic D3 deformation phase. The anatectic leucogranites are coeval to slightly younger than the adjacent intrusive granodiorites and monzogranites (305.4±1.6Ma for Refugio del Rey leucogranites and 303±2Ma for migmatitic hornfelses). It is suggested that these anatectic magmas were generated in response to the thermal effects of granodiorite intrusions. Thermal modeling with COMSOL Multiphysics® reveals that sequential emplacement was able to keep the thermal conditions of the batholith around the temperature of zircon crystallization in granitic melts (around 750°C) for several million of years, favoring the partial melting of host rocks and the existence of large magma chambers composed of crystal mush prone to be rejuvenated after new intrusions.

Application of ASTER remote sensing data to geological mapping of basement domains in arid regions: a case study from the Central Anti-Atlas, Iguerda inlier, Morocco

Satellite remote sensing is shown to provide critical support for geological and structural mapping in semiarid and arid areas. In this work, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to clarify the geological framework of the Precambrian basement of the Iguerda Proterozoic inlier in the Moroccan Central Anti-Atlas. In this study, the interpretation of the processed digital data has been ground truthed with geological field data collected during a reconnaissance-mapping program in the Central Anti-Atlas. The Iguerda inlier offers a deeply eroded Precambrian massif dominated by a Paleoproterozoic basement composed of supracrustal metasedimentary units intruded by various Eburnian granitoids. Impressive mafic dyke swarms mainly of Proterozoic age crosscut this basement. Eburnian basement rocks are unconformably overlain by Lower Ediacaran volcanosedimentary rocks of the Ouarzazate Group and Upper Ediacaran–Lower Cambrian carbonates. The applied ASTER analyses are particularly effective in the lithological differentiation and discrimination of geological units of the Iguerda inlier. The spectral information divergence (SID) classification algorithm coupled with spectral angle mapper and maximum likelihood classification effectively discriminates between metamorphic rocks, granitoid bodies, and carbonate cover. SID classification improves geologic map accuracy with respect to the spatial distribution of plutonic bodies and metamorphic units. In addition, Paleoproterozoic granitoids have been well discriminated into separate distinct suites of porphyritic granites, granodiorites, and peraluminous leucogranite suites. This discrimination was initially identified via remote sensing analysis and later ground truthed in the field. This methodology enhances geological mapping and illustrates the potential of ASTER data to serve as a vital tool in detailed geologic mapping and exploration of well-exposed basement of arid regions, such as the Proterozoic of the Anti-Atlas Mountains of Morocco.