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

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.

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

2019 ◽  
Vol 132 (5-6) ◽  
pp. 1257-1272 ◽  
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.

Geochemical constraints on the evolution of the early continental crust

1981 ◽  
Vol 301 (1461) ◽  
pp. 423-442 ◽  
Keyword(s):  
Trace Element ◽  
Continental Crust ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Tectonic Setting ◽  
Mantle Plumes ◽  
Element Abundances ◽  
Ocean Island ◽  
Rock Types ◽  
Greenstone Belts

The most important process affecting both major and trace-element concentrations in the mantle and crust is melting producing silicate liquids which then migrate. Another process whose effects are becoming more apparent is the transport of elements by CO 2 - and H 2 O-rich fluids. Due to the relatively small amounts of fluids involved they have but little effect on the major-element abundances but may severely affect minor- and trace-element abundances in their source and the material through which they travel. The Archaean crust was a density filter which reduced the possibility of komatiite or high FeO melts with relative densities greater than about 3.0 from reaching the surface. Those melts retained in the lower crust or at the crust-mantle boundary would have enhanced the possibility of melting in the lower crust. The high FeO melts may have included the Archaean equivalents of alkali basalt whose derivatives may form an important component in the Archaean crust. The occurrence of ultramafic to basic to alkaline magmas in some Archaean greenstone belts is an assemblage most typical of modern ocean-island suites in continental environments. The rock types in the assemblage were modified by conditions of higher heat production during the Archaean and thus greater extents of melting and melting at greater depths. If modern ocean-island suites are associated with mantle plumes, which even now may be an important way to transport heat upward from the deeper mantle, it is suggested that during the Archaean mantle plumes were an important factor in the evolution of the continental crust. It appears that the Archaean continental crust was of comparable thickness to that of the present based on geobarometeric data. If the freeboard concept applied then, this would suggest that plate tectonics was also an active process during the Archaean. If so, it is probably no more realistic to assume that all Archaean greenstone belts had a similar tectonic setting than to assume that all modern occurrences of basic rocks have a common tectonic setting.

scholarly journals Identification and Geological Significance of Early Jurassic Adakitic Volcanic Rocks in Xintaimen Area, Western Liaoning

Minerals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 331
Author(s):  
Zhi-Wei Song ◽  
Chang-Qing Zheng ◽  
Chen-Yue Liang ◽  
Bo Lin ◽  
Xue-Chun Xu ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
North China Craton ◽  
Lower Crust ◽  
North China ◽  
Basaltic Magma ◽  
Volcanic Rocks ◽  
Early Jurassic ◽  
Magma Source ◽  
Western Liaoning

The Western Liaoning area, where a large number of Jurassic-Cretaceous volcanic rocks are exposed, is one of the typical areas for studying the Mesozoic Paleo-Pacific and Mongolia-Okhotsk subduction process, and lithospheric destruction of North China Craton. The identification and investigation of Early Jurassic adakitic volcanic rocks in the Xintaimen area of Western Liaoning is of particular significance for exploring the volcanic magma source and its composition evolution, tracking the crust-mantle interaction, and revealing the craton destruction and the subduction of oceanic plates. Detailed petrography, zircon U–Pb dating, geochemistry, and zircon Hf isotope studies indicate that the Early Jurassic intermediate-acidic volcanic rocks are mainly composed of trachydacites and a few rhyolites with the formation ages of 178.6–181.9 Ma. Geochemical characteristics show that they have a high content of SiO2, MgO, Al2O3, and total-alkali, typical of the high-K calc-alkaline series. They also show enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), depletion of heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and have a high content of Sr and low content of Y and Yb, suggesting that they were derived from the partial melting of the lower crust. The εHf(t) values of dated zircons and two-stage model ages (TDM2) vary from −11.6 to −7.4 and from 1692 to 1958 Ma, respectively. During the Early Jurassic, the study area was under long-range tectonic effects with the closure of the Mongolia-Okhotsk Ocean and the subduction of the Paleo-Pacific plate, which caused the basaltic magma to invade the lower crust of the North China Craton. The mantle-derived magma was separated and crystallized while heating the Proterozoic lower crust, and part of the thickened crust melted to form these intermediate-acidic adakitic volcanic rocks.

scholarly journals Heterogeneity in lunar anorthosite meteorites: implications for the lunar magma ocean model

2014 ◽  
Vol 372 (2024) ◽  
pp. 20130241 ◽  
Author(s):  
Sara S. Russell ◽  
Katherine H. Joy ◽  
Teresa E. Jeffries ◽  
Guy J. Consolmagno ◽  
Anton Kearsley
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Ocean Model ◽  
Magma Source ◽  
The Moon ◽  
Magma Ocean ◽  
Element Abundances ◽  
Wide Range ◽  
Lunar Meteorites

The lunar magma ocean model is a well-established theory of the early evolution of the Moon. By this model, the Moon was initially largely molten and the anorthositic crust that now covers much of the lunar surface directly crystallized from this enormous magma source. We are undertaking a study of the geochemical characteristics of anorthosites from lunar meteorites to test this model. Rare earth and other element abundances have been measured in situ in relict anorthosite clasts from two feldspathic lunar meteorites: Dhofar 908 and Dhofar 081. The rare earth elements were present in abundances of approximately 0.1 to approximately 10× chondritic (CI) abundance. Every plagioclase exhibited a positive Eu-anomaly, with Eu abundances of up to approximately 20×CI. Calculations of the melt in equilibrium with anorthite show that it apparently crystallized from a magma that was unfractionated with respect to rare earth elements and ranged in abundance from 8 to 80×CI. Comparisons of our data with other lunar meteorites and Apollo samples suggest that there is notable heterogeneity in the trace element abundances of lunar anorthosites, suggesting these samples did not all crystallize from a common magma source. Compositional and isotopic data from other authors also suggest that lunar anorthosites are chemically heterogeneous and have a wide range of ages. These observations may support other models of crust formation on the Moon or suggest that there are complexities in the lunar magma ocean scenario to allow for multiple generations of anorthosite formation.

scholarly journals Paleoproterozoic Adakitic Rocks in Qingchengzi District, Northeastern Jiao-Liao-Ji Belt: Implications for Petrogenesis and Tectonism

Minerals ◽  
2020 ◽  
Vol 10 (8) ◽  
pp. 684
Author(s):  
Jian Li ◽  
Hanlun Liu ◽  
Keyong Wang ◽  
Wenyan Cai
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Tectonic Setting ◽  
Active Continental Margin ◽  
Biotite Granite ◽  
Granitic Rocks ◽  
Geochemical Data ◽  
Trace Element Geochemistry ◽  
Element Geochemistry ◽  
Magmatic Rocks

Herein, zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock major and trace element geochemistry are presented for two Palaeoproterozoic granitic rocks in Qingchengzi district, northeastern Jiao-Liao-Ji Belt (JLJB). These new geochronological and geochemical data provide reference clues for exploring the petrogenesis and tectonic setting of Paleoproterozoic magmatic rocks in the Qingchengzi district, which further constrain the tectonic nature of the JLJB. Our zircon U-Pb dating denotes that the Paleoproterozoic magmatic events in the Qingchengzi district were emplaced at ~2163 Ma and ~1854 Ma, represented by granite porphyry and biotite granite, respectively. Geochemically, these Palaeoproterozoic rocks are characterized by high Sr (760–842 ppm), SiO2 (69.72–70.89 wt.%), and Al2O3 (15.53–16.78 wt.%) contents, low Y (2.1–9.0 ppm) and Yb (0.25–0.80 ppm) contents, which indicate an adakite affinity. Combined with Hf isotopic composition (εHf(t) = −1.5~+4.8; TDM2 = 3109~2560 Ma), we believe that the Paleoproterozoic adakitic magma originated from partial melting of the thickened lower crust material in the Meso-Neoarchean. Moreover, these rocks are enriched in light rare earth elements and large ion lithophilic elements (e.g., K, Rb, and Cs), and depleted in heavy rare earth elements and high field strength elements (e.g., Nb and Ta). These features are similar to magmatic rocks formed in an arc environment (either island arc or active continental margin) and are not consistent with an intraplate/intracontinental environment. According to this study and previous research results, we conclude that the arc–continent collision model is conducive to the Paleoproterozoic tectonic attribute of the JLJB, and the oceanic crust subduction between the Namgrim and Longgang blocks may have induced the widespread occurrence of magmatic events in the region.

The petrogenesis of metamorphosed carbonatites in the Grenville Province, Ontario

1997 ◽  
Vol 34 (9) ◽  
pp. 1185-1201 ◽  
Author(s):  
David P. Moecher ◽  
Eric D. Anderson ◽  
Claudia A. Cook ◽  
Klaus Mezger
Keyword(s):  
Rare Earth ◽  
Mantle Source ◽  
Earth Element ◽  
Minor Element ◽  
Geochemical Data ◽  
Boundary Zone ◽  
Grenville Province ◽  
Nd Isotope ◽  
Element Abundances ◽  
Depleted Mantle

Veins and dikes of calcite-rich rocks within the Central Metasedimentary Belt boundary zone (CMBbz) in the Grenville Province of Ontario have been interpreted to be true carbonatites or to be pseudocarbonatites derived from interaction of pegmatite melts and regional Grenville marble. The putative carbonatites have been metamorphosed and consist mainly of calcite, biotite, and apatite with lesser amounts of clinopyroxene, magnetite, allanite, zircon, titanite, cerite, celestite, and barite. The rocks have high P and rare earth element (REE) contents, and calcite in carbonatite has elevated Sr, Fe, and Mn contents relative to Grenville Supergroup marble and marble mélange. Values of δ18OSMOW (9.9–13.3‰) and δ13CPDB (−4.8 to −1.9‰) for calcite are also distinct from those for marble and most marble mélange. Titanites extracted from clinopyroxene–calcite–scapolite skarns formed by metasomatic interaction of carbonatites and silicate lithologies yield U–Pb ages of 1085 to 1035 Ma. Zircon from one carbonatite body yields a U–Pb age of 1089 ± 5 Ma; zircon ages from two other bodies are 1170 ± 3 and 1143 ± 8 Ma, suggesting several carbonatite formation events or remobilization of carbonatite during deformation and metamorphism around 1080 Ma. Values of εNd(T) are 1.7–3.2 for carbonatites, −1.5–1.0 for REE-rich granite dikes intruding the CMBbz, and 1.6–1.7 for marble. The mineralogy and geochemical data are consistent with derivation of the carbonatites from a depleted mantle source. Mixing calculations indicate that interaction of REE-rich pegmatites with regional marbles cannot reproduce selected major and minor element abundances, REE contents, and O and Nd isotope compositions of the carbonatites.

Granitic batholiths: from pervasive and continuous melting in the lower crust to discontinuous and spaced plutonism in the upper crust

2006 ◽  
Vol 97 (4) ◽  
pp. 311-324 ◽  
Author(s):  
Jean Louis Vigneresse
Keyword(s):  
Lower Crust ◽  
Magma Mixing ◽  
Tectonic Setting ◽  
Continuous Process ◽  
Chemical Compositions ◽  
Upper Crust ◽  
Linear Processes ◽  
Non Linear ◽  
Thermally Driven ◽  
Two Phases

ABSTRACTThe generation of granitic magmas begins with melting in the lower crust, under active participation of the underlying mantle. Thermally driven, melting is a pervasive and continuous process that develops over a wide region. In contrast, the building of a granitic pluton is highly discontinuous in time and space. Several inputs of magma, sometimes with a different chemical compositions, are focused toward a region where they accumulate, forming a large pluton, often separated by some 50 km from an adjacent one. The switch from a continuous to a discontinuous process represents a fundamental point of magma generation. It gives place to the modified model m(M-SAE), in which the mantle (m) and Melting (M) are separated from the Segregation (S), Ascent (A) and Emplacement (E) modes. Discontinuities result from non-linear processes that develop during segregation and ascent of the magma. They rely on the non-linear rheology of partially molten rocks. Thresholds control the change from a solid-like to liquid-like behaviour of the magma. In between, the rheology exhibits sudden jumps between states. Because two phases continuously coexist (matrix and melt), strain is highly partitioned between them. This may induce highly discontinuous melt segregation, which needs both pure and simple shear to develop. Melt focusing is controlled by the viscosity contrast between the two phases. It gives rise to different compaction lengths depending on the region, a partially melting source or a nearly brittle crust, where it develops. Because ascent and emplacement are discontinuous in time, this allows the crust to relax, avoiding the room problem for a pluton intruding the upper crust. Intermediate magma chambers could develop with different temperature and magma composition. They could be the place of enhanced magma mixing. Finally, the stress conditions, which differ for each tectonic setting, influence the shape of the granitic body.

Miocene adakites in south Tibet: Partial melting of the thickened Lhasa juvenile mafic lower crust with the involvement of ancient Indian continental crust compositions

2019 ◽  
Vol 132 (5-6) ◽  
pp. 1273-1290
Author(s):  
Haoyu Yan ◽  
Xiaoping Long ◽  
Jie Li ◽  
Qiang Wang ◽  
Xuan-Ce Wang ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Continental Crust ◽  
Lower Crust ◽  
Adakitic Rocks ◽  
Magma Sources ◽  
Mafic Lower Crust ◽  
Porphyritic Texture ◽  
South Tibet

Abstract Although postcollisional adakitic rocks are widely distributed in the southern Lhasa subterrane, their petrogenesis remains controversial. Complex petrogenesis models, mainly including partial melting of subducted oceanic crust, partial melting of the Indian lower continental crust, and magma mixing, are pivotal in reconstruction of the postcollisional dynamic processes in south Tibet. In order to constrain the geodynamic processes, we present systemic geochronological and geochemical data for newly discovered adakitic dikes in the Xigaze area, southern Lhasa subterrane. Based on the K2O and Na2O contents, the Xigaze dikes can be divided into K-rich and Na-rich dikes. Zircon U-Pb dating for the Xigaze K- and Na-rich dikes yielded ages of ca. 10.31 Ma and 14.78–12.75 Ma, respectively. The K-rich dikes show porphyritic texture and are characterized by high SiO2 (68.91–69.59 wt%) and K2O (5.53–5.68 wt%) contents and low Na2O/K2O (0.48–0.60) ratios, with Al2O3/(CaO + Na2O + K2O) (=A/CNK) ratios of 1.07–1.23. They have lower MgO (0.63–0.64 wt%), Mg# (37–39), and Cr (18.56–26.62 ppm) and Ni (4.37–4.62) contents. In addition, the K-rich dikes display enriched ([La/Yb]N = 65–68) light rare earth elements (LREEs), low concentrations of heavy rare earth elements (HREEs) and Y (e.g., Yb = 0.83–0.86 ppm; Y = 10.56–11.55 ppm), and high Sr (841–923 ppm), with high Sr/Y (74–84) ratios, indicating geochemical characteristics of typical adakitic rocks. Compared with the K-rich dikes, the Na-rich dikes also display porphyritic texture, but they have lower SiO2 (59.14–64.87 wt%) and K2O (1.98–3.25 wt%) contents, and higher Na2O (4.43–5.64 wt%) and MgO (1.40–3.08 wt%) contents, Mg# (46–59), and Cr (22.62–82.93 ppm) and Ni (8.91–39.76 ppm) contents. The HREE abundances (e.g., Yb = 0.36–0.81 ppm; Y = 5.30–10.56 ppm) of the Na-rich dikes are generally lower than the K-rich dikes. These Na-rich dikes are also characterized by adakitic geochemical features with high Sr/Y (60–223) but low (La/Yb)N (15–40) ratios. Both the K-rich and Na-rich dikes display distinct whole-rock-element geochemistry and Sr-Nd isotopic composition, with (87Sr/86Sr)i = 0.7121, εNd(t) = –8.62 to –8.11 and (87Sr/86Sr)i = 0.7054–0.7086, εNd(t) = –7.55 to –1.23 for K-rich and Na-rich dikes, respectively, which indicate different magma sources for the two types of dikes. The K-rich dikes were most likely derived from partial melts of Lhasa juvenile mafic lower crust with significant involvement of Indian continental crust compositions, whereas the Na-rich dikes were generated in the same way with less input of Indian continental crust compositions. Moreover, the postcollisional adakites in the southern Lhasa subterrane display distinctive spatial variations in geochemistry along the strike of this subterrane, indicating that the magma sources were heterogeneous. In combination with previously published data, we therefore suggest that all these late Oligocene to Miocene adakitic rocks were most likely generated dominantly by partial melting of the Lhasa mafic lower crust with involvement of Indian continental crust components, which was probably triggered by the tearing of the subducting Indian plate.

scholarly journals Petrogenesis and Tectonic Setting of the Highly Fractionated Junye Granitic Intrusion in the Yiliu Tungsten Polymetallic Deposit, Guangdong Province, South China: Constraints from Geochemistry and Sr-Nd-Pb-Hf Isotopes

Minerals ◽  
2020 ◽  
Vol 10 (7) ◽  
pp. 631
Author(s):  
Tao Wu ◽  
Zhilong Huang ◽  
Mu Yang ◽  
Dexian Zhang ◽  
Jiawei Zhang ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
South China ◽  
Tectonic Setting ◽  
Granitic Magma ◽  
Geochemical Data ◽  
Metallogenic Province ◽  
Polymetallic Deposit ◽  
South Central ◽  
High K

The Yiliu tungsten polymetallic deposit, located in the south central portion of the Nanling nonferrous metal metallogenic province in South China, is an area with common Yanshanian tectonothermal events. Early Yanshanian magmatism leads to the emplacement of voluminous tungsten-bearing granite intrusions, such as the Baoshan, Benggangling and Junye plutons, which are considered temporally and spatially associated with W-polymetallic mineralization in the Yiliu region. Here, we investigate the basic geological and petrological characteristics of the Junye granites, and present major and trace element geochemical data and bulk-rock Sr-Nd-Pb-Hf isotopic data to gain insight into the petrogenesis and tectonic setting of granitic intrusions in the region. The Junye granites are high-K calc-alkaline and metaluminous to weakly peraluminous [A/CNK = molar ratios of Al2O3/(CaO + Na2O + K2O) = 0.97–1.02] with enrichment in SiO2 (75.68–76.44 wt.%), relatively high total alkalis (K2O + Na2O = 8.06–8.45 wt.%) with K2O/Na2O ratios ranging from 1.12 to 1.42, and moderate Al2O3 (12.62–13.00 wt.%), but low in P2O5 (<0.01 wt.%), MgO (0.02–0.04 wt.%), CaO (0.78–0.95 wt.%) and Fe2O3T (0.93–1.07 wt.%). They show spectacular tetrad effect REE (rare earth element) patterns with low ΣREE content (53.2–145.3 ppm), negative Eu anomalies (δEu = 0.09–0.17) and slight enrichment of LREEs (light rare earth elements) relative to HREEs (heavy rare earth elements). The granites are enriched in Rb (481–860 ppm), Th (16.2–46.1 ppm) and U (25.4–40.8 ppm) but depleted in Ba (1.0–5.8 ppm), Sr (11.1–23.4 ppm), P (9.5–26.7 ppm) and Ti (241–393 ppm). All geochemical features lead us to interpret the Junye granites as highly fractionated I-type granites. These granites underwent intense interaction between highly evolved magma and volatile-rich hydrothermal fluids during the late stage of formation, and accompanied fractional crystallization of biotite, plagioclase and accessory minerals, such as apatite, monazite and allanite. Additionally, the granites show uniform Nd isotopic ratios with calculated εNd (152 Ma) values of −8.28 to −8.91 and Nd model age (TDM2) of 1645 to 1698 Ma, stable age-corrected initial Pb isotopic compositions with (206Pb/204Pb)i of 18.646–19.010, (207Pb/204Pb)i of 15.767–15.786 and (208Pb/204Pb)i of 39.113–39.159, respectively, and homogeneous Hf isotopic values yielding εHf (152 Ma) values from −6.9 to −9.5 with TDM2 ages of 1680 to 2214 Ma, collectively suggesting that the granitic magma was probably derived from the remelting of ancient infracrustal materials in the basement of the Nanling region. Consequently, we consider that the Junye granites are the products of partial melting of Paleoproterozoic infracrustal medium- to high-K metamorphic basaltic rocks in the Cathaysia Block, which was caused by the underplating of coeval mantle basaltic magmas that provided abundant heat energy for melting in a tectonic setting, with lithospheric extension and thinning during the late Jurassic period.

Mineral chemistry, petrogenesis, and tectonic setting of the Wateranga layered intrusion, southeast Queensland, Australia

2005 ◽  
Vol 42 (11) ◽  
pp. 1967-1985 ◽  
Author(s):  
Reddy VR Talusani ◽  
Warwick J Sivell ◽  
Paul M Ashley
Keyword(s):  
Tectonic Setting ◽  
Parental Magma ◽  
Layered Intrusion ◽  
Mafic Magma ◽  
Mineral Chemistry ◽  
Geochemical Data ◽  
Magma Source ◽  
Olivine Gabbro ◽  
Layered Mafic Intrusion ◽  
Southeast Queensland

The Wateranga layered mafic intrusion (28 km2 in area, > 500 m thick) is a tholeiitic, undeformed, unmetamorphosed, Permo-Triassic layered gabbroic pluton intruded into the late Carboniferous Goodnight beds of the Goodnight Block in southeast Queensland. The intrusion mainly consists of gabbro and norite, associated with subordinate amounts of troctolite, anorthosite, and orthopyroxenite, and rare picrite. Olivine gabbro is the dominant rock type of the intrusion. Fractionation followed a tholeiitic trend with iron enrichment in the liquid. Petrographic, mineral chemical, and whole-rock geochemical data have been used to divide the intrusion into Lower, Middle, and Upper zones, which are interpreted as reflecting magma chamber replenishment. The observed changes in the crystallization order between the zones reveal that a single parental magma is inadequate to explain the data. The common differentiation indices, such as An content of plagioclase, Mg#s of olivine, clinopyroxene, orthopyroxene and whole-rocks, and the whole-rock concentrations of various incompatible trace elements (Zr, Y, Nb, La Ba, Rb, Sr, and Nd), all vary widely with stratigraphic depth and display abrupt shifts at the zone boundaries, indicating open system addition of new mafic magma. Temperatures estimated from two-pyroxene geothermometer vary from 1057 to 927 °C. During the course of crystallization, pressure probably was > 2 and < 4 kbar (1 kbar = 100 MPa). The variation trend of anorthite content of plagioclase versus the forsterite content of olivine precludes an arc-related magma source. The composition and geological setting of the intrusion are consistent with emplacement in a post-subduction extensional tectonic environment.

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