Low Heavy Rare Earth Research Articles

TTG generation by fluid-fluxed crustal melting: Direct evidence from the Proterozoic Georgetown Inlier, NE Australia

Across the Archaean to Proterozoic transition, the composition of newly-formed felsic continental crust changed from tonalite–trondhjemite–granodiorite (TTG) to calc-alkaline granitoid, possibly coinciding with the emergence of plate tectonics. Nevertheless, TTG suites were sporadically produced in Proterozoic and Phanerozoic orogenic belts, and such occurrences may provide petrological and tectonic insights into the formation of ancient continents. Here we demonstrate that the ca 1560 Ma Forest Home TTG plutonic suite in the Georgetown Inlier, NE Australia, was derived from partial melting of spatially-associated mafic rocks in a post-collisional setting. The studied TTG rocks have a ‘high-pressure’ geochemical signature, with elevated Sr, low heavy rare earth element and low high field strength element contents. Established petrogenetic models suggest they were derived by partial melting either of hydrated basaltic crust at >70 km depth or enriched lithospheric mantle, or by fractionation of lower-pressure mafic magmas. Using phase equilibrium calculations and trace-element modelling, we show that the geochemical signature of the Georgetown TTG likely resulted from fluid-fluxed crustal melting at relatively shallow depths (25–35 km), consistent with field observations and the inferred metamorphic evolution of the inlier. Our results suggest that the chemical variability of TTGs can reflect the variable availability of fluids rather than depth of melting, which has implications for tectonic processes responsible for the formation of early continental crust.

The origin of low-MgO eclogite xenoliths from Obnazhennaya kimberlite, Siberian craton

The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11–16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure–temperature conditions 2.3–3.7 GPa and 855–1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like δ18O of the garnets (5.07–5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N < 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy REE patterns ((Dy/Yb)N < 1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We, therefore, suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures, where garnet was the main residual phase.

Adakitic rocks and A‐type felsic dykes in the Changlingzi area, NE China: Constraints on multistage tectonism in the southern Great Xing'an Range

New LA–ICP–MS zircon U–Pb geochronology, whole‐rock major and trace element geochemistry data, and petrographic observations are presented for three granitic dykes in the Changlingzi area of NE China. These data provide precise age and petrogenesis information with respect to the late Palaeozoic–Mesozoic intrusions in the area, which further constrain the geodynamic evolution of the southern Great Xing'an Range. Our zircon U–Pb data denote that the felsic dykes were emplaced during the Late Permian (~254 Ma) and Late Jurassic (143–142 Ma). Geochemically, the Late Permian monzonite porphyries are characterized by high Al2O3 (16.52–16.92 wt%) and Sr (490–579 ppm) contents and low Yb (0.52–0.57 ppm), Y (5.7–6.3 ppm), and heavy rare earth element (HREE) contents, which indicate an adakitic affinity. The Late Jurassic felsic dykes are characterized by high FeOt/MgO (8.2–11.9) and Ga/Al (3.3–3.8) ratios, high zircon saturation temperatures (888–929°C), and remarkable negative Eu (Eu/Eu* = 0.25–0.42) anomalies, indicating they are akin to A‐type granites. The Late Permian adakitic magma resulted from the partial melting of thickened lower crust, and was formed during collisional orogenesis related to subduction of Palaeo‐Asian Ocean crust. The Late Jurassic A‐type magmas were probably generated by the partial melting of amphibolite‐facies lower crustal materials. The melting occurred in response to the upwelling of asthenospheric material caused by collapse of thickened crust, due to gravitational instability during closure of the Mongol–Okhotsk Ocean. Subsequently, the latest stage of magmatic activity in this area and elsewhere in NE China was controlled mainly by subduction of Palaeo‐Pacific Ocean crust.

Geodynamic Evolution of Flat‐Slab Subduction of Paleo‐Pacific Plate: Constraints From Jurassic Adakitic Lavas in the Hailar Basin, NE China

AbstractSubduction of the Paleo‐Pacific Plate caused widespread Mesozoic magmatism, lithospheric deformation and thinning, and mineralization along the continental margin of East Asia. However, the details of this subduction process remain unclear. To investigate the regional geodynamic evolution and subduction processes of the Paleo‐Pacific Plate, we carried out a geochronological, geochemical, and Sr‐Nd‐Hf isotopic study of Middle‐Late Jurassic adakitic lavas in the Hailar Basin, northeast (NE) China. These rocks have relatively high Sr contents (434–877 ppm), low heavy rare earth element and Y (6.81–14.37 ppm) contents, and high Sr/Y ratios (56–83), which are characteristic of adakites. The rocks are classified into Middle Jurassic high‐K and Late Jurassic low‐K adakitic lavas. The high‐K adakitic lavas were derived by dehydration melting of thickened garnet‐bearing amphibolite‐facies lower crust. In contrast, the low‐K adakitic lavas could be considered as pristine slab‐melts that were likely generated by hydrous melting of subducted eclogitic oceanic crust, followed by limited interaction between the slab melt and peridotite as a consequence of flat‐slab subduction. Combined with previous data, our results demonstrate that the late Mesozoic magmatism in NE China records the entire temporal‐spatial evolution of subduction of the Paleo‐Pacific slab from initial shallowing of the subduction angle to flat‐slab subduction, followed by basalt‐eclogite phase transformation and finally slab rollback.

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

AbstractAbundant 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.

Paleoproterozoic (~2.0 Ga) volcano‐plutonism in the southeastern region of the Amazon Craton: Petrological aspects and geotectonic implications

Detailed studies on the volcano‐plutonic associations of the Amazon Craton (AC) have been conducted since the 1980s. Since then, several issues concerning the definition of the lithostratigraphic units and their role in the AC evolution have been improved. Recently, a volcano‐plutonic association of around 2.0 Ga was described in the south‐central part of the AC, named as Jarinã Formation (volcanic rocks) and Vila Rica Intrusive Suite (granitoids). The present work deals with the unexpected occurrence of this volcano‐plutonic association in the southeastern region of the Amazon Craton. In the study area (Santaninha region), the rocks of the Jarinã Formation comprise two volcanic successions with distinct petrographic and compositional features. The amount and shape of the phenocrysts and crystal fragments allow the individualization of effusive and pyroclastic rocks. The geochemical character indicates that the volcanic rocks belong to two distinct successions as follows: intermediate to acid magnesian I‐type and acid ferroan A‐type. The I‐type succession shows high MgO, TiO2, P2O5, and Sr content and low K2O, Zr, Nb, Ta, Y, and heavy rare earth element and consists of undeformed andesitic to dacitic ignimbrites and subordinate effusive rhyodacites. The A‐type succession present high K2O, FeOt, Zr, Nb, Ta, Y, and heavy rare earth element content and low TiO2, P2O5, and Sr and comprises undeformed rhyolitic ignimbrites and subordinate effusive rhyolites. The granitoids of the Vila Rica Intrusive Suite crosscut the volcanic rocks of the Jarinã Formation and include two phaneritic facies as follows: coarse‐ to medium‐grained granite and medium‐grained granite. The MgO, TiO2, P2O5, Sr, K2O, Zr, Nb, Ta, Y, and rare earth element content demonstrate that the medium‐grained granite facies present geochemical characteristic of I‐type granites. Laser‐ablation inductively coupled plasma mass spectrometry U–Pb zircon data indicate crystallization ages of 2,009 ± 9 Ma for the I‐type dacitic ignimbrite, 1,994 ± 16 Ma for the A‐type rhyolitic ignimbrite, and 1,995 ± 14 Ma for the I‐type medium‐grained granite. These ages agree with the previous data of the Jarinã Formation in the south‐central region of the Amazon Craton and of the Vilca Rica Intrusive Suite in the southeastern of the Amazon Craton. The understanding of the geological context of these units carry on meaningful implications to the contextualization of the ~2.0 Ga volcano‐plutonic event in the AC and, consequently, to the compartimentalization of its geotectonic provinces.

Different melting conditions and petrogenesis of peraluminous granites in western Qinling, China, and tectonic implications

The Qinling orogen records the prolonged amalgamation history between the North China Block (NCB) and the South China Block (SCB) accompanied by major granitoid intrusions during the Triassic. However, the petrogenesis of these granitoids, especially that of the peraluminous granites, remains a matter of debate. Detailed study of these peraluminous granites, which are the products of crustal melting, will give insights into the melting process and petrogenesis of peraluminous magmas and will further constrain the regional geological evolution. In this study, Xiahe (XH) tourmaline-bearing two-mica granite and Baijiazhuang (BJZ) two-mica granite were selected for systematic analysis of their mineralogy, petrology, zircon U-Pb geochronology, and geochemistry. The zircon U-Pb dating showed that the XH and BJZ granites were emplaced at approximately 242 Ma and approximately 214 Ma, respectively. The presence of muscovite and tourmaline, high peraluminosity at >1.1, low oxygen fugacity (ƒO2) conditions below the fayalite–magnetite–quartz (FMQ) buffer, and low zircon saturation temperatures indicate sedimentary protolith (S-type) granitic affinity. The low CaO/Na2O ratios <0.4 suggest that the granites formed by partial melting of a pelitic source. The low heavy rare earth element (HREE) concentration and weak negative Eu anomaly in the XH granite implies garnet residue in the magma source, whereas an obvious Eu anomaly in the BJZ granite indicates no residual garnet. The Rb/Sr ratios remained constant in the XH granite with decreasing Sr and Ba concentrations; however, these ratios increased in the BJZ granite. This observation reflects that the XH (BJZ) granite was the product of fluid-present (fluid-absent) muscovite melting. This finding is in accord with the different geochemical features and the estimates of zircon and monazite saturation temperature between the XH and BJZ granites. In western Qinling, low-Sr/high-REE (group A) and high-Sr/low-REE (group B) peraluminous granites have been identified. We propose that the former were generated during fluid-absent muscovite melting, whereas the latter were derived from fluid-present muscovite melting. This hypothesis was further confirmed by higher zircon and monazite saturation temperature estimates in addition to higher REE, Th, Nb, and Ta concentrations in the group A granitoids. From regional geological considerations, it can be concluded that the Middle Triassic granitoids in western Qinling formed during subduction of the A'nimaqing–Mianlue oceanic crust, whereas the Late Triassic granitoids are related to syn- or post-collisional processes.

Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction

Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon UPb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon UPb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns to E-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock εNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock εNd(t) (+2.9 to +3.4) and zircon εHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.

Linking high-pressure mafic granulite, TTG-like (tonalitic-trondhjemitic) leucosome and pluton, and crustal growth during continental collision

Research Article| November 09, 2018 Linking high-pressure mafic granulite, TTG-like (tonalitic-trondhjemitic) leucosome and pluton, and crustal growth during continental collision Shengyao Yu; Shengyao Yu † 1Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China2Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China †yushengyao@ouc.edu.cn Search for other works by this author on: GSW Google Scholar Sanzhong Li; Sanzhong Li 1Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China2Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China Search for other works by this author on: GSW Google Scholar Jianxin Zhang; Jianxin Zhang 3Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Search for other works by this author on: GSW Google Scholar Deyou Sun; Deyou Sun 4College of Earth Sciences, Jilin University, Changchun 130061, China Search for other works by this author on: GSW Google Scholar Yinbiao Peng; Yinbiao Peng 1Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China Search for other works by this author on: GSW Google Scholar Yunshuai Li Yunshuai Li 5Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China Search for other works by this author on: GSW Google Scholar GSA Bulletin (2019) 131 (3-4): 572–586. https://doi.org/10.1130/B31822.1 Article history received: 06 Apr 2017 rev-recd: 10 Jan 2018 accepted: 14 Aug 2018 first online: 10 Nov 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shengyao Yu, Sanzhong Li, Jianxin Zhang, Deyou Sun, Yinbiao Peng, Yunshuai Li; Linking high-pressure mafic granulite, TTG-like (tonalitic-trondhjemitic) leucosome and pluton, and crustal growth during continental collision. GSA Bulletin 2018;; 131 (3-4): 572–586. doi: https://doi.org/10.1130/B31822.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The early continental crust is composed predominantly of Archean tonalite-trondhjemite-granodiorite (TTG); however, the tectonic regime and melting conditions for TTG magmas have been debated. In this study, we report field and microscopic evidence for the partial melting of arc-related metagabbro and its products, including TTG-like melt, volumetrically significant plutons evolved from melt, and the associated granulitic residua during continental collision in the North Qaidam Mountains, China. Migmatite shows successive stages of initial intragranular or droplet-like melt along grain boundaries, which grew into a three-dimensional interconnected intergranular network, segregated, and accumulated in pressure shadow areas, and merged to form melt channels and sheets that finally combined to form a TTG-like tonalite pluton. Pressure-temperature (P-T) calculations indicate high-pressure granulite-facies metamorphism and the crystallization of leucosome at P = 15.5–18.5 kbar and T = 850–950 °C. Based on zircon U-Pb dating and petrological analyses, partial melting and magmatic crystallization occurred 438–430 m.y. ago, which is slightly younger or temporally overlaps with ultrahigh-pressure metamorphism in the region. The metagabbros exhibit a subduction-related arc signature with slightly positive εNd(t) values of 2.1–4.2. The felsic leucosomes and tonalite plutons are characterized by high Na, Sr, Sr/Y, and La/Yb values and low heavy rare earth element values, with εNd(t) values of 0.1–4.3, similar to typical TTGs. The geological context, geochemistry, and timing of the TTG-like melt formation observed in this study differ from the prevailing models; however, our observations and documentations demonstrate that melting of arc-like metagabbro under high-pressure granulite-facies conditions during continental collision may make important contributions to crustal growth and differentiation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemistry of peridotites from the Yap Trench, Western Pacific: implications for subduction zone mantle evolution

ABSTRACTThis study examines the major and trace elements of peridotites from the Yap Trench in the western Pacific to investigate mantle evolution beneath a subduction zone. Major element results show that the peridotites are low in Al2O3 (0.31–0.65 wt.%) and CaO (0.04–0.07 wt.%) contents and high in Mg# (Mg/(Mg+Fe)) (0.91–0.92) and have spinels with Cr# (Cr/(Cr+Al)) higher than 0.6 (0.61–0.73). Trace element results show that the peridotites have extremely low heavy rare earth element (HREE) contents compared with abyssal peridotites but have U-shaped chondrite-normalized rare earth element (REE) patterns. The degree of mantle melting estimated based on the major elements, HREEs, and spinel Cr# range from 19% to 25%, indicating that the Yap Trench peridotites may be residues of melting associated with the presence of water in the mantle source. In addition to light rare earth element (LREE) enrichment, the peridotites are characterized by high contents of highly incompatible elements, positive U and Sr anomalies, negative Ti anomalies, and high Zr/Hf ratios. The correlations between these elements and both the degree of serpentinization and high field strength element (HFSE) contents suggest that fluid alteration alone cannot account for the enrichment of the peridotites and that at least the enrichment of LREEs was likely caused by melt–mantle interaction. Comparison between the peridotites and the depletion trend defined by the primitive mantle (PM) and the depleted mantle (DM) suggests that the Yap Trench mantle was modified by subduction-related melt characterized by high contents of incompatible elements, high Zr/Hf ratios, and low HFSE contents. Hydrous melting may have been enhanced by tectonic erosion of the subducting Caroline Plate with complex tectonic morphostructures at the earliest stages of subduction initiation.

Eclogites and garnet pyroxenites from Kimberley, Kaapvaal craton, South Africa: their diverse origins and complex metasomatic signatures

We describe the petrography and mineral chemistry of sixteen eclogite and garnet pyroxenite xenoliths from the reworked Boshof road dump (Kimberley) and define three groups that stem from different depths. Group A, the shallowest derived, has low HREE (heavy rare earth element) abundances, flat middle to heavy REE patterns and high Mg# [= 100·Mg/(Mg + Fe)]. Their protoliths probably were higher pressure cumulates (~ 0.7 GPa) of mainly clinopyroxene (cpx) and subordinate orthopyroxene (opx) and olivine (ol). Group B1 xenoliths, derived from the graphite/diamond boundary and below show similarities to present-day N-MORB that were modified by partial melting (higher Mg# and positively inclined MREE (middle REE) and HREE (heavy REE) patterns of calculated bulk rocks). Group B2 samples from greatest depth are unique amongst eclogites reported so far worldwide. The calculated bulk rocks have humped REE patterns with very low La and Lu and prominent maxima at Sm or Eu and anomalously high Na2O (up to 5 wt%) which makes protolith identification difficult. The complex trace element signatures of the full spectrum of Kimberley eclogites belie a multi-stage history of melt depletion and metasomatism with the introduction of new phases especially of phlogopite (phlog). Phlogopite appears to be characteristic for Kimberley eclogites and garnet peridotites. Modelling the metasomatic overprint indicates that groups A and B1 were overprinted by volatile- and potassium-rich melts probably by a process of chromatographic fractionation. Using constraints from other metasomatized Kimberley mantle rocks suggest that much of the metasomatic phlogopite in the eclogites formed during an intense episode of metasomatism that affected the mantle beneath this region 1.1 Gyr ago.

Petrogenesis of Early Cretaceous dioritic dikes in the Shanyang-Zhashui area, South Qinling, central China: Evidence for partial melting of thickened lower continental crust

The dioritic dikes distributed in the Shanyang-Zhashui area of the South Qinling region play an important role in understanding the deep magmatic processes and tectonic evolution during the orogenic process. The zircon UPb ages of the dioritic dikes indicate that they were emplaced at ∼144 Ma and therefore postdate the dikes that formed in the intracontinental orogenic background after the continental collision between the North China Block (NCB) and the South China Block (SCB). The dioritic dikes have SiO2 contents of 56.86–64.93 wt%; K2O contents of 1.65–3.21 wt%; low MgO (1.50–2.66 wt%), Y (14.4–25.5 ppm) and heavy rare earth element (HREE) contents; low Mg# values (39.9–49); high Sr contents (528–4833 ppm); and high Sr/Y ratios (32.8–189). They exhibit highly fractionated REE and flat HREE patterns, strong enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, and U) and depletion in high field strength elements (HFSEs) (e.g., Nb), as well as positive Sr and negative Ti anomalies. Furthermore, these dioritic dikes exhibit (87Sr/86Sr)i ratios ranging from 0.7048 to 0.7083, εNd(t) values ranging from −3.3 to −1.4, and εHf(t) values ranging from −4.1 to 1.6. The geochemical patterns of the dioritic dikes indicate that they possess adakitic characteristics. Moreover, the low MgO contents, Mg# values, Ni contents, Th/Ce ratios, and SrNdHf isotopic features all indicate that these dioritic dikes were generated by the partial melting of thickened mafic lower crust. The high La/Yb and Sr/Y ratios, low Y and Yb contents, absence of significant Eu anomalies, flat HREE patterns, and low Nb/Ta ratios of these rocks suggest that the adakitic melts were derived from the melting of garnet-bearing amphibolite. The geochronologic, elemental and isotopic evidence suggests that the dioritic dikes may have formed in a locally extensional environment within an overall N-S compressional setting or during the transition from compressional to extensional environments in the Early Cretaceous. This process resulted in the upwelling of the asthenospheric or lithospheric mantle, causing partial melting of the mafic lower crust and forming the adakitic dioritic melts.

Coercivity enhancement of sintered Nd-Fe-B magnets by chemical bath deposition

The chemical bath deposition (CBD) method is used to diffuse the heavy rare earth element in order to obtain the high coercivity magnets with low heavy rare earth element. The jet mill powders are soaked in the alcohol suspension of Dy(CH3CHOHCH3)3 (Dy-ipa) so that Nd2Fe14B powder particles are surrounded by Dy-ipa homogeneously. By adding 1.0 wt. % Dy, the coercivity of magnet is increased from 14.47 kOe to 17.55 kOe with slight reduction of remanence after grain boundary diffusion (GBD) in the sintering and annealing processes. The temperature coefficient of coercivity optimizes from -0.629 %/°C to -0.605 %/°C as well as that of remanence improves from -0.108 %/°C to -0.100 %/°C. The CBD method is helpful for thermal stability and alignment either. The relation between the microstructure and the coercivity has been studied systematically.

Precise microbeam dating defines three Archaean granitoid suites at the southwestern margin of the Kaapvaal Craton

Precise microbeam U-Pb zircon dates have been determined for 17 granitoid samples which crop out along the southwestern margin of the Kaapvaal Craton. In the Marydale High two main types of granitoid are distinguished mainly by their normative Quartz – Alkali Feldspar – Plagioclase mineral proportions. The Draghoender Granite type is generally tonalitic to trondhjemitic and some samples have very low heavy Rare Earth Elements, probably originating as melts of eclogitic protoliths. Their ages vary by ∼50 Ma from 2946 ± 9 Ma to 2892 ± 6 Ma (2σ), reflecting a long period of magmatism possibly due to subduction. The granodioritic to monzogranitic Skalkseput Granite type intrudes the Draghoender granite in places and has a coherent age of 2901 ± 14 Ma. These two granite types thus overlap in time and space. The more evolved Skalkseputs type could have been derived by melting of the Draghoender type during assembly of the Kaapvaal Craton by collision of the Kimberley and Witwatersrand Terranes.A third granite type is exemplified by the 2721 ± 6 Ma monzogranitic Steenkop Granite Gneiss which occurs south of Prieskapoort, and corresponds in age to a 2718 ± 8 Ma monzogranite dyke which intrudes a 2871 Ma Skalkseput monzogranite. The previous age of 2718 ± 8 Ma for the Skalkseput Granite probably also dated a Steenkop-type dyke. A basal conglomerate to the Zeekoebaard Formation of the Ventersdorp Supergroup has a single 2720 ± 4 Ma detrital zircon population.South of the Marydale High a narrow strip of Draghoender-type Granite extends to Franzenhof where it is exposed in a Dwyka glacial outwash surface and is dated at 2931 ± 9 Ma. The southernmost exposures of granite are windows in the Dwyka group about 130 km southeast of Marydale on both sides of the Doornberg Fault. The 2905 ± 7 Ma Maritzdam Granite which is basement to Ventersdorp Supergroup volcanic rocks at Kuip is monzogranitic and corresponds to the Skalkseput type. On the southwest side of the Doornberg Fault the 2907 ± 4 Ma Welgevonden Granite underlies Marydale Group volcanic rocks, showing that the Kaapvaal basement extends across the fault.

Large-scale removal of lithosphere underneath the North China Craton in the Early Cretaceous: Geochemical constraints from volcanic lavas in the Bohai Bay Basin

Cratons are generally considered as the most stable tectonic units on the Earth. Rare magmatism, seismic activity, and intracrustal ductile deformation occur in them. However, several cratons experienced entirely different fates, including the North China Craton (NCC), and were subsequently destroyed. Geodynamic mechanisms and timing of the cratonic destruction are strongly debated. In this paper, we investigate a suite of Mesozoic intermediate to felsic volcanic rocks which are collected from boreholes in the Liaohe Depression of the Bohai Bay Basin the eastern NCC. These volcanic rocks have Precambrian basement-like Sr–Nd isotopic characteristics, consistent with derivation from the lower continental crust underneath the NCC. The Late Jurassic (~165Ma) intermediate volcanic rocks don't exhibit markedly negative Eu anomalies, which require a source beyond the plagioclase stability field. And the low heavy rare earth elements (HREEs) contents of these samples indicate that their source has garnet as residue. The Early Cretaceous (~122Ma) felsic volcanic rocks are depleted in HREEs but with remarkable Eu anomalies, suggesting that their source have both garnet and plagioclase. The crust thicknesses, estimated from the geochemistry of the intermediate and felsic rocks, are ≥50km at ~165Ma and 30–50km at ~122Ma, respectively. The crustal thinning is attributed to lithospheric delamination beneath the NCC. Our results combined with previous studies imply that the large-scale lithospheric removal occurred in the Early Cretaceous, between 140 and 120Ma.

An evolving tectonic environment of Late Carboniferous to Early Permian granitic plutons in the Chinese Altai and Eastern Junggar terranes, Central Asian Orogenic Belt, NW China

The Central Asian Orogenic Belt (CAOB) represents one of the most important sites of juvenile crustal growth during the Phanerozoic. Located in the central part of the CAOB, the Chinese Altai and Eastern Junggar terranes record the collisional processes between the peri-Siberian and Kazakhstan orogenic systems. However, the precise timing of collision between the two terranes remains controversial. The Wukuli and Kadelat plutons in the Chinese Altai belt are dated at ∼305 and ∼280Ma respectively, whereas the Aketas pluton in the Eastern Junggar terrane is dated at ∼308Ma. Granites from the Wukuli and Kadelat plutons are strongly peraluminous (A/CNK>1.1), and are characterized by low Al2O3, Na2O, MnO, MgO, CaO and heavy rare earth element (HREE) contents, but with high SiO2, K2O and Rb contents as well as high Rb/Sr ratios. Granites from the Wukuli pluton have low εNd(t) and εHf(t) values of −3.7 to −3.4 and −9.7 to +4.9, whereas those from the Kadelat pluton have values of −3.6 to −3.4 and −8.0 to +2.6. These features suggest S-type affinity for the Wukuli and Kadelat plutons with magma derivation through partial melting of Mesoproterozoic metasediments. The Aketas pluton is composed of weakly peraluminous quartz monzonites that have A/CNK values ranging from 0.92 to 1.08, with high Na2O, Sr, and Sr/Y, and low Y, Yb, Nb, and Ta. These rocks display positive εNd(t) (+4.8 to +6.4) and εHf(t) (+9.7 to +14.6) values, and low initial 87Sr/86Sr ratios (0.703357–0.703868), similar to modern adakites, suggesting that the quartz monzonites were derived from the partial melting of lower crustal material. The geochemical characteristics suggest that the Aketas pluton was formed in a subduction-related setting, the Wukuli pluton in a syn-collisional setting, and the Kadelat pluton in the subsequent post-orogenic strike-slip-related setting. In combination with data from other granitoids in these two terranes, the Aketas pluton represents the youngest record of subduction-related environments, suggesting that the final collision between the Chinese Altai and Eastern Junggar terranes might have occurred between 308 and 304Ma.

High Sr/Y Magma Petrogenesis and the Link to Porphyry Mineralization as Revealed by Garnet-Bearing I-Type Granodiorite Porphyries of the Middle Cauca Au-Cu Belt, Colombia

Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y > 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchia district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, garnet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y. Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of >1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa. Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y 40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intramineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries. The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur.

Late Jurassic, high Ba–Sr Linglong granites in the Jiaodong Peninsula, East China: lower crustal melting products in the eastern North China Craton

AbstractThe Jurassic Linglong granites, intrusive into the North China Craton (NCC) in eastern China, provide a critical record of the first major episode of lithospheric-scale extension and magmatism in NE China during Mesozoic time. Our U–Pb zircon dating reveals that the Linglong granites were emplaced during 161–158 Ma, shortly after the inception of a shallow subduction of the Palaeo-Pacific plate beneath East Asia during Middle Jurassic time. These granites have high alkali contents (K2O + Na2O = 8–9 wt%), low MgO and Mg no. values and variable Cr–Ni abundances. Their relatively high Ba and Sr concentrations, relatively low heavy rare Earth element (HREE) and strongly fractionated REE patterns characterize them as high Ba–Sr granites. The negative whole-rockεNd(t) values ranging from −22.4 to −10.9 and wide-ranging zirconεHf(t) values of −39.1 to −1.5 suggest that magmas of the Linglong granites were produced by partial melting of a garnet-amphibolite-bearing lower crust of the Jiaobei Terrane and by re-melting of the Triassic ultrahigh-pressure (UHP) metamorphic rocks and alkaline suites of the Sulu Terrane. The occurrence in the granitic rocks of inherited zircons of the Neoarchaean, Palaeoproterozoic, Neoproterozoic, Palaeozoic and Triassic ages suggests that magmas of the Linglong granites interacted with the ancient crust in these terranes during their ascent. Asthenospheric upwelling, induced by the steepening and rapid rollback of the Palaeo-Pacific slab during Late Jurassic time, provided the heat source for the inferred lower crustal melting. Trench migration and thermal weakening of the crust caused extensional deformation and thinning in the eastern part of the NCC.

B isotopes of Carboniferous‐Permian volcanic rocks in the Tuha basin mirror a transition from subduction to intraplate setting in Central Asian Orogenic Belt

AbstractControversies remain as to the Permian tectonic setting in the Central Asian Orogenic Belt (CAOB), in particular, regarding the triggering of Permian magmatism. To address this issue, we carried out a geochemical study on Dananhu volcanics from the Tuha basin, southwestern CAOB. 40Ar/39Ar analyses for feldspar separates yield a Carboniferous age of 321.2 ± 9.8 Ma for the andesites and a Permian age of 278.9 ± 4.2 Ma for the basalts. Both the andesites and basalts geochemically resemble subduction‐related magmas in aspects of their trace element and Sr‐Nd‐Pb isotopic composition, but they may differ in petrogenesis. The andesites are typical adakites. Their high Cr and Ni contents, and low heavy rare earth element contents, and positive correlation between B/Nb ratios and δ11B values suggest that they were generated by interaction between slab‐derived melts/fluids and the mantle wedge. The Permian basalts are subdivided into two subgroups: alkali and tholeiitic basalts. High Ba/La and Ba/Zr ratios indicate that the source of the alkali basalts has been metasomatized by fluids/melts derived from altered oceanic crust. In contrast, a volatile‐free source metasomatized by sediment‐derived fluids/melts is inferred for the tholeiitic basalts given their high Th/Zr and Th/Ce ratios. Negative correlation between B/Nb and δ11B for the Permian basalts imply that the arc signatures were imposed by previous subduction events, and no direct slab‐derived fluids/melts participated in their petrogenesis. These geochemical characteristics alongside regional geologic records collectively suggest a tectonic shift in the southwestern CAOB from subduction‐related setting during the Late Carboniferous to an intraplate setting during the Early Permian.

Primary Silica-rich Picrite and High-Ca Boninite Melt Inclusions in Pyroxenite Veins from the Kamchatka Sub-arc Mantle

Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (>15 wt % MgO, 48–54 wt % SiO2) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (>25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ∼15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.