Whole-rock Nd–Hf isotopic study of I-type and peraluminous granitic rocks from the Chinese Altai: Constraints on the nature of the lower crust and tectonic setting

Abstract As a result of the evolution of Meso-Tethys, Early Cretaceous granitoids are widespread in the eastern Tengchong terrane, SW China, which is considered as the southern extension of the Tibetan Plateau. These igneous rocks are therefore very important for understanding the tectonic setting of Meso-Tethys and the formation of the Tibetan Plateau. In this paper, we present new zircon U-Pb dating, whole-rock elemental, and Nd isotopic data of granitoids obtained from the eastern Tengchong terrane. Our results show that these granitoids are composed of monzogranites and granodiorites and formed at ca. 124 Ma in the Early Cretaceous. Mineralogically and geochemically, these granitoids display metaluminous nature and affinity to I-type granites, which are derived from preexisting intracrustal igneous source rocks. The predominantly negative whole-rock εNd(t) values (−10.86 to −8.64) for all samples indicate that they are derived mainly from the partial melting of the Mesoproterozoic metabasic rocks in the lower crust. Integrating previous studies with the data presented in this contribution, we propose that the Early Cretaceous granitic rocks (135–110 Ma) also belong to I-type granites with minor high fractionation. Furthermore, in discriminant diagrams for source, granitoids are mainly derived from the partial melting of metaigneous rocks with minor sediments in the lower crust. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar and mafic enclaves indicate that these Cretaceous igneous rocks were the products of the tectonic evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab rollback. Moreover, the Tengchong terrane is probably the southern extension of the South Qiangtang terrane.

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Himalayan and Trans Himalayan granitic rocks in and adjacent to Nepal and their mineral potential

Journal of Nepal Geological Society ◽

10.3126/jngs.v1i1.32668 ◽

1981 ◽

Vol 1 (1) ◽

Author(s):

A. H. G. Mitchell

Keyword(s):

Root Zone ◽

Porphyry Copper ◽

Tectonic Setting ◽

Granitic Rocks ◽

Main Central Thrust ◽

Southern Tibet ◽

Late Mesozoic ◽

The North ◽

Augen Gneiss ◽

Host Rocks

Granitic rocks occupying eight distinct tectonic settings can be recognized in the Himalayas and Transhimalayas. In the Lower Himalayas geographical belt a few plutons of two-mica granite intrude the lowest unit of the Nawakot Complex or Midland Group. More extensive are sheet- like lies of augen gneiss intrusive within a possibly thrust bounded succession carbonates and graphitic schists beneath the Main Central Thrust to the north. The most abundant granites in the Lower Himalayas are the two- mica cordierite- bearing granite within klippen; minor tin and tungsten mineralization is associated with these plutons, which are of late Cambrian age. Within the Higher Himalayas above the Main Central Thrust, the ‘Central Crystallines’ or Central Gneisses include pegmatites and pegmatitic granites intrusive into gneisses of probable early Proterozoic age; these have same potential for ruby, sapphire, aquamarine and possibly spodumene. Further north within the Higher Himalayan succession a southern belt of anatectic two- mica granites and leucogranites of mid-Tertiary age is favorable for tin, tungsten and uranium mineralization; a northern belt of granites or gneisses is of uncertain age and origin. North of the Indus Suture in the Transhimalayas extensive batholiths of hornblende granodiorite representing the root zone of a late Mesozoic to early Eocene volcanic arc are associated with porphyry copper deposits. Further north in southern Tibet the tectonic, setting for reported granitic bodies of Tertiary age is uncertain; their location suggests that they could be favorable host rocks for tin, uranium and porphyry molybdenum mineralization.

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Melt segregation in the lower crust: how have experiments helped us?

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s0263593300006490 ◽

1996 ◽

Vol 87 (1-2) ◽

pp. 73-83 ◽

Cited By ~ 23

Author(s):

Tracy Rushmer

Keyword(s):

Lower Crust ◽

Tectonic Setting ◽

Experimental Studies ◽

Crustal Rock ◽

Melt Migration ◽

Tectonic Environment ◽

Rock Cores ◽

Melt Distribution ◽

Chemical Behaviour ◽

Melt Segregation

ABSTRACT:The rheological and chemical behaviour of the lower crust during anatexis has been a major focus of geological investigations for many years. Modern studies of crustal evolution require significant knowledge, not only of the potential source regions for granites, but also of the transport paths and emplacement mechanisms operating during granite genesis. We have gained significant insights into the segregation and transport of granitoid melts from the results of experimental studies on rock behaviour during partial melting. Experiments performed on crustal rock cores under both hydrostatic conditions and during deformation have led, in part, to two conclusions. (1) The interfacial energy controlling melt distribution is anisotropic and, as a result, the textures deviate significantly from those predicted for ideal systems—planar solid-melt interfaces are developed in addition to triple junction melt pockets. The ideal dihedral angle model for melt distribution cannot be used as a constraint to predict melt migration in the lower crust. (2) The ‘critical melt fraction’ model, which requires viscous, granitic melt to remain in the source until melt fractions reach >25 vol%, is not a reliable model for melt segregation. The most recent experimental results on crustal rock cores which have helped advance our understanding of melt segregation processes have shown that melt segregation is controlled by several variables, including the depth of melting, the type of reaction and the volume change associated with that reaction. Larger scale processes such as tectonic environment determine the rate at which the lower crust heats and deforms, thus the tectonic setting controls the melt fraction at which segregation takes place, in addition to the pressure and temperature of the potential melting reactions. Melt migration therefore can occur at a variety of different melt fractions depending on the tectonic environment; these results have significant implications for the predicted geochemistry of the magmas themselves.

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Comparative studies on two phases of Archaean TTG magmas from different blocks of the North China Craton: petrogenesis and constraints on crustal evolution

Geological Magazine ◽

10.1017/s0016756820000618 ◽

2020 ◽

pp. 1-16

Author(s):

Houxiang Shan ◽

Mingguo Zhai ◽

RN Mitchell ◽

Fu Liu ◽

Jinghui Guo

Keyword(s):

North China Craton ◽

Lower Crust ◽

North China ◽

Tectonic Setting ◽

Source Material ◽

Crustal Growth ◽

Garnet Amphibolite ◽

The North ◽

Western Block ◽

Subducting Slab

Abstract Whole-rock major and trace elements and Hf isotopes of magmatic zircons of tonalite–trondhjemite–granodiorite (TTG) rocks with different ages (2.9, 2.7 and 2.5 Ga) from the three blocks (the Eastern Block, Western Block and Trans-North China Orogen) of the North China Craton were compiled to investigate their respective petrogenesis, tectonic setting and implications for crustal growth and evolution. Geochemical features of the 2.5 Ga TTGs of the Eastern Block require melting of predominant rutile-bearing eclogite and subordinate garnet-amphibolite at higher pressure, while the source material of the 2.7 Ga TTGs is garnet-amphibolite or granulite at lower pressure. The 2.5 Ga TTGs have high Mg#, Cr and Ni, negative Nb–Ta anomalies and a juvenile basaltic crustal source, indicating derivation from the melting of a subducting slab. In contrast, features of the 2.7 Ga TTGs suggest generation from melting of thickened lower crust. The 2.5 and 2.7 Ga TTGs in the Trans-North China Orogen were formed at garnet-amphibolite to eclogite facies, and the source material of the 2.5 Ga TTGs in the Western Block is most likely garnet-amphibolite or eclogite. The 2.5 Ga TTGs in the Trans-North China Orogen and Western Block were generated by the melting of a subducting slab, whereas the 2.7 Ga TTGs in the Trans-North China Orogen derived from melting of thickened lower crust. The Hf isotopic data suggest both the 2.5 and 2.7 Ga TTG magmas were involved with contemporary crustal growth and reworking. The two-stage model age (TDM2) histograms show major crustal growth between 2.9 and 2.7 Ga for the whole North China Craton.

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Geochronological and Geochemical Constraints on the Origin of the Hutouya Polymetallic Skarn Deposit in the East Kunlun Orogenic Belt, NW China

Minerals ◽

10.3390/min10121136 ◽

2020 ◽

Vol 10 (12) ◽

pp. 1136

Author(s):

Hongchang Gao ◽

Fengyue Sun ◽

Bile Li ◽

Ye Qian ◽

Li Wang ◽

...

Keyword(s):

Lower Crust ◽

Inductively Coupled Plasma ◽

Orogenic Belt ◽

Granitic Rocks ◽

Skarn Deposit ◽

Nw China ◽

Inductively Coupled ◽

East Kunlun ◽

Ore Minerals ◽

East Kunlun Orogenic Belt

The Hutouya polymetallic skarn deposit lies in the Qimantagh area of the East Kunlun Orogenic Belt, NW China. Skarnization and mineralization at the deposit are closely associated with contemporary felsic intrusions. In this paper, zircon U-Pb ages and zircon Hf isotope as well as whole-rock geochemical and whole-rock Sr-Nd isotope data are reported for intrusive rocks and crystal tuff of the Elashan Formation in the Hutouya area. Moreover, Re-Os ages and S-Pb isotopes are also reported for the ore minerals in the Hutouya deposit. The Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U-Pb age of granodiorite and Re-Os isochron age of molybdenite suggest that mineralizations occurred at ca. 227 Ma and that the granodiorite and molybdenite are closely related petrogenetically. All the granitoids in the Hutouya deposit are high-K calc-alkaline and metaluminous to weakly peraluminous I-type granitoids. Among them, the ore-forming granitoids were derived by the mixing of crust-derived (either juvenile or ancient mature lower crust) and mantle-derived magmas, whereas the non-ore-related granite porphyry was generated by the partial melting of a single ancient mature lower crust. The magmas of all the granitoids underwent extensive fractionation–crystallization during the process of rising and emplacement. The sulfur of the analyzed samples from the northern and middle zone of Hutouya deposit (including No. II, III, IV, and VI ore belts) belongs to deep magmatic sulfur, while the sulfur of samples from the southern zone of Hutouya deposit (No. VII ore belt) includes not only deep magmatic sulfur but also a contribution of strata sulfur. All the ore mineral samples in the Hutouya deposit have similar Pb compositions that are consistently derived from a mixed source of upper crust and mantle. Tectonic discrimination diagrams indicate a post-collisional setting for all granitic rocks of the Hutouya skarn deposit, which is therefore considered a product of a the post-collision extensional system and is consistent with other porphyry-skarn deposits within the East Kunlun Orogenic Belt.

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Geochemistry and Sr- and Nd-isotopic study on rare-metal-bearing granitic rocks, central Eastern Desert, Egypt

Precambrian Research ◽

10.1016/0301-9268(95)00099-2 ◽

1996 ◽

Vol 80 (1-2) ◽

pp. 1-22 ◽

Cited By ~ 23

Author(s):

M Hassanen

Keyword(s):

Eastern Desert ◽

Rare Metal ◽

Granitic Rocks ◽

Isotopic Study ◽

Central Eastern Desert ◽

Central Eastern

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Two phases of post-onset collision adakitic magmatism in the southern Lhasa subterrane, Tibet, and their tectonic implications

Geological Society of America Bulletin ◽

10.1130/b35326.1 ◽

2019 ◽

Vol 132 (7-8) ◽

pp. 1587-1602

Author(s):

Tian-Yu Lu ◽

Zhen-Yu He ◽

Reiner Klemd

Keyword(s):

Rare Earth ◽

Lower Crust ◽

Tectonic Setting ◽

Geochemical Data ◽

Magma Source ◽

New Mineral ◽

Element Abundances ◽

Lhasa Terrane ◽

Adakitic Rocks ◽

Two Phases

Abstract Abundant Neogene adakitic magmatism occurred in the southern Lhasa subterrane after the onset of the India–Asia collision while convergence continued. However, the tectonic setting and magmatic evolution of the adakitic rocks are still under discussion. This study includes new mineral chemical and whole-rock geochemical data as well as zircon U-Pb and Lu-Hf isotopes of adakitic intrusive rocks from the Gyaca and Nyemo locations in the southern Lhasa subterrane. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U-Pb dating yielded crystallization ages of ca. 30 Ma for the Gyaca and Nyemo monzogranite and ca. 15 Ma for the Nyemo granodiorite. Both have common chemical signatures such as low MgO and heavy rare earth element contents as well as low compatible element abundances, indicating that these rocks result from partial melting of thickened lower crust with residual eclogite and garnet amphibolite. Furthermore, these rocks are characterized by variable positive zircon εHf(t) values, suggesting a juvenile magma source with variable ancient crustal contributions. Taking previous data into account, the adakitic magmatism concurs with an early late Eocene to Oligocene (ca. 38–25 Ma) and a late Miocene (ca. 20–10 Ma) phase. The adakitic rocks of the two phases are characterized by different fractionation evolutions of light and medium rare earth elements. We propose that the early-phase adakitic rocks were generated by the anatexis of Lhasa terrane lower crust owing to crustal shortening and thickening subsequent to the onset of the India–Asia collision and the upwelling of hot asthenosphere beneath the Lhasa terrane caused by the break-off of the Neo-Tethyan oceanic slab. The latest phase of adakitic rocks, however, relates to upwelling asthenosphere following the delamination and/or break-off of the subducting Indian continental slab.

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Generation of the 105–100 Ma Dagze volcanic rocks in the north Lhasa Terrane by lower crustal melting at different temperature and depth: Implications for tectonic transition

Geological Society of America Bulletin ◽

10.1130/b35306.1 ◽

2019 ◽

Vol 132 (5-6) ◽

pp. 1257-1272 ◽

Cited By ~ 5

Author(s):

Yun-Chuan Zeng ◽

Ji-Feng Xu ◽

Feng Huang ◽

Ming-Jian Li ◽

Qin Chen

Keyword(s):

Early Cretaceous ◽

Lower Crust ◽

Tectonic Setting ◽

Volcanic Rocks ◽

Geochemical Data ◽

Lhasa Terrane ◽

The North ◽

Tectonic Transition ◽

Group I ◽

Group Ii

Abstract Successively erupted intermediate-felsic rocks with variations in their geochemical compositions indicate physical changes in lower-crust conditions, and the variations can provide important insights into the regional tectonic setting. What triggered the late Early Cretaceous tectonic transition of the central-north Lhasa Terrane remains controversial, hindering the understanding of the mechanisms behind the formation of the central Tibetan Plateau. The sodic Dagze volcanic rocks in the north Lhasa Terrane are characterized by high contents of SiO2 and Na2O, low contents of MgO, Fe2O3, and K2O, and low values of Mg#. However, the trace element compositions of the whole-rocks and their zircons allow the rocks to be divided into two groups. The Group I rocks (ca. 105 Ma) have higher contents of Sr and Ba, higher Sr/Y and La/Yb ratios, and lower contents of Y, Yb, Ti, and Zr than Group II rocks (ca. 100 Ma). Besides, the zircons from Group I rocks have higher values of Yb/Gd and U/Yb, lower values of Th/U, and lower Ti contents than the zircons from Group II rocks. However, the rocks of both groups have identical depleted whole-rock Sr-Nd and zircon Hf isotope values. The geochemical data indicate that rocks of both groups were generated by partial melting of a juvenile lower crust, but the differences in the two groups reflect a transition from deep-cold melting to relatively shallower-hotter melting in the period from ca. 105 to 100 Ma. This transition was synchronous with the rapid cooling of granitoids, topographic uplift, and the shutdown of magmatism in the central-north Lhasa Terrane, and followed by sedimentation and the resumption of magmatism in the south Lhasa Terrane. The above observations collectively indicate that the central-north Lhasa Terrane was under an extensional setting in late Early Cretaceous, and we tentatively suggest that it was in response to lithospheric drip during roll-back of the northward-subducting Neo-Tethyan oceanic plate.

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A Rb–Sr isotopic study of the Archean rocks of the eastern Lac Seul region, English River subprovince, northwestern Ontario

Canadian Journal of Earth Sciences ◽

10.1139/e80-056 ◽

1980 ◽

Vol 17 (5) ◽

pp. 569-576 ◽

Cited By ~ 6

Author(s):

Joseph L. Wooden ◽

Alan M. Goodwin

Keyword(s):

Source Region ◽

Granitic Rocks ◽

Isotopic Study ◽

Zircon Age ◽

Crustal Rocks ◽

Northwestern Ontario ◽

Plutonic Complex ◽

History Of ◽

Lower Crustal ◽

Good Agreement

Rb–Sr whole-rock data for the gneissic and granitic rocks of the eastern Lac Seul region, when combined with the U–Pb zircon dating of Krogh, document a history of multiple intrusion for the area. The oldest rocks are the Sen Bay plutonic complex gneisses which have complex Rb–Sr systematics. Interpretation of the Rb–Sr data yields model ages of 3000–3100 Ma which are in good agreement with a zircon age of 3040 Ma. The next oldest rocks are trondhjemitic–granodioritic gneisses with a Rb–Sr age of 2780 ± 90 Ma. The initial Sr ratio (I) of 0.7009 ± 4 for these rocks suggests that this age approximates the time of intrusion and that the magma was derived from lower crustal rocks with a very short residence lime in the crust. Following a period of deformation and metamorphism, granodioritic to granitic dikes, sills, and small plutons were intruded between 2660 and 2560 Ma ago. I values for these racks range from 0.7019–0.7027. If the I values of these rocks represent the source region for the granitic magmas, then one explanation for the I values would be that the magmas were derived from a source region of mixed lithology and age. The Sen Bay plutonic complex is considered to represent an earlier cycle of crustal formation which is distinct from a later 2800–2550 Ma old cycle which dominates much of the Superior Province.

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Petrogenesis of Volcanic Arc Granites from Bayah Complex, Banten, Indonesia

Journal of Geoscience Engineering Environment and Technology ◽

10.25299/jgeet.2019.4.2.3171 ◽

2019 ◽

Vol 4 (2) ◽

pp. 104

Author(s):

Jemi Saputra Ahnaf ◽

Aton Patonah ◽

Haryadi Permana

Keyword(s):

Tectonic Setting ◽

Crustal Contamination ◽

Polarized Light ◽

Granitic Rocks ◽

Tectonic Activity ◽

Molar Ratio ◽

Petrographic Analysis ◽

Volcanic Arc ◽

Regional Tectonic ◽

Type Granite

This research aimed to reveal the petrogenesis of granitic rocks of Bayah Complex starting from magma differentiation to exposing event, this research also intended to determine the tectonic environment. The methods carried out in this research include field observation, petrographic analysis using polarized light microscopy, and geochemical analysis using X-Ray Fluorescence (XRF) and Inductively Coupled Mass Spectrometry (ICP-MS). Petrographic analysis shows that Bayah granitic rocks are composed of quartz, plagioclase, and K-feldspar while the rest are amphibole, biotite, sericite, chlorite, epidote, and opaque. Based on its major oxide concentrations, Bayah granitic rocks classified as granite and diorite-quartz which have high-K calc-alkaline magma. 4 samples of granitic rocks showed the A/N+K+C > 1 molar ratios belonging to the peraluminous S-type granite index while the remaining 1 sample showed a molar ratio of A/N+ K+C < 1 and A/N+K > 1 which classified as metaluminous I-type granite. Accordingly, Bayah granitic rocks are S-type granite which crystallized from sediment-derived magma, the sediments itself estimated sourced from continental especially Malay Peninsula, Indonesian Tin Island, and Schwaner Mountains. During differentiation, the magma undergone crustal contamination reflected by the increase in both SiO2 0.51 wt% and Al2O3 1.95 wt%, and decrease in Fe2O3 + MgO 0.61 wt% from the pure composition of sediment-derived magma. Furthermore, the occurrence of crustal contamination also recognized from high concentrations of Rb and Ba which indicate the interaction of magma with the materials of continental crust. Regard to the exposing event, Bayah granitic rocks approximated to be exposed due to regional tectonic activity which caused Orogenesa I in the Early Oligocene to the Late Oligocene. Moreover, based on the plot of trace elements especially Rb, Y, Nb, Ta, and Yb on Harker and tectonic discriminant diagrams, Bayah granitic rocks are formed on volcanic-arc active continental margins in accordance with regional tectonic setting.

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