Heavy Rare Earth Element Abundances Research Articles

Origin of the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, deduced from the trace element characteristics of clinopyroxene

The origin of basal lherzolites in the mantle section of harzburgite‐dominated ophiolites is enigmatic. The basal part of the mantle section is well exposed in the Muslim Bagh Ophiolite, Pakistan, which is one of the harzburgite‐dominated ophiolites of the Tethys Ophiolite Belt. In this contribution, we describe the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, and discuss its origin based on the trace‐element characteristics of its clinopyroxenes. The basal lherzolite exhibits porphyroclastic to mylonitic textures. Primitive mantle‐normalized trace‐element patterns of the porphyroclastic clinopyroxenes were characterized by low ratios of light rare‐earth elements (LREEs) to heavy rare‐earth elements (HREEs), low abundances of HREEs, and positive Sr anomalies. These geochemical characteristics are not consistent with a lherzolite and its clinopyroxenes, which is formed by a residue after a low degree of partial melting and melt extraction from peridotites in a mid‐ocean ridge setting. Instead, the compositions of the clinopyroxenes are consistent with open‐system melting induced by the infiltration of slab‐derived fluids into residual peridotites that had been depleted in REEs. The compositions of chromian spinels in the chromitites of the basal peridotite sequence are also consistent with their formation in an arc setting. We conclude that the basal lherzolites of the Muslim Bagh Ophiolite represent a residue after a relatively high degree of partial melting, and that the clinopyroxenes were added as metasomatic crystallization from slab‐derived arc‐related melts to this residual depleted peridotite in a subduction setting.

Rhönite in Cenozoic alkali basalt from Changle, Shandong Province, China, and its significance

Abstract. Four Cenozoic, rhönite-bearing alkali-olivine basalt samples from the Changle area (Shandong Province, China) show an intracontinental character and were generated in an extensional setting. Petrographic studies document different generations of rhönite. In three samples, rhönite occurs either as a reaction product surrounding coarse-grained corundum, spinel and phlogopite or along cleavage planes in phlogopite. In one sample rhönite forms disseminated crystals in a mantle xenolith, possibly formed by a reaction of coarse-grained orthopyroxene or spinel with a melt. Rhönite exhibits a wide range of compositions: 22.9 wt %–33.0 wt % SiO2, 13.3 wt %–19.0 wt % Al2O3, 9.4 wt %–19.9 wt % MgO and 10.210.2 wt %–24.5 wt % FeO. The derived primary substitutions include (1) SiIV + NaVII = (Al, Fe3+)IV + CaVII, (2) MgVI = (Fe2+, Mn2+)VI and (3) TiVI + (Mg + Fe2+ + Mn2+)VI = 2Fe3+VI. Rare-earth-element (REE) patterns of euhedral rhönite crystals from the mantle xenolith (sample SS17) and those surrounding spinel (sample CL04) have a concave-upward shape for the heavy rare-earth elements (HREEs) and are slightly enriched in the light rare-earth elements (LREEs). These patterns resemble those of kaersutitic amphibole and kaersutite reported from basanite, olivine nephelinite, transitional alkali-olivine basalt and hawaiite. In contrast, REE patterns of the other two samples containing fine-grained, anhedral and acicular rhönite crystals (samples CL01 and EGS03) are relatively steep, with lower HREE and higher LREE abundances, similar to those of ocean island basalts (OIBs). All types of Changle rhönite show positive Nb, Ti and V anomalies in spidergrams normalized to primitive mantle. Mineral assemblages of the studied samples indicate that rhönite crystallized at different stages within a temperature range from about 950 to 1180 ∘C and at pressures below 0.5 kbar, with fO2 below the NNO buffer. The chemical composition of Changle rhönite is interpreted to depend on the composition of the initial silicate melt, the redox conditions during crystallization and the composition of the minerals involved in reactions to form rhönite. Similar to metasomatic mantle amphibole, the compositions of Changle rhönites cover the I-Amph (I-amphibole) and S-Amph (S-amphibole) fields, indicating that they may have formed due to an intraplate metasomatic event, overprinting an older metasomatic subduction episode.

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

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.

Are Clay Minerals the Primary Control on the Oceanic Rare Earth Element Budget?

The rare earth elements (REEs) are an important tool for understanding biogeochemical cycling and sedimentary processes in the global ocean. However, ambiguities in the marine REE budgets, including questions around the dominant source of REEs to the ocean, hinder the application of this tool. A bottom-up model for REE release into the ocean has recently been proposed, driven by early diagenetic processes such as sediment dissolution, with potentially significant implications for the interpretation of marine REE and Nd isotope paleo-records. Here, our goal is to identify the phase or phases that interact with the pore fluids to drive such a benthic flux. We use new pore water REE, microbeam imaging and mineralogical data in combination with published pore water REE data to evaluate potential sedimentary REE host phases. Mineralogical and direct imaging observations suggest that authigenic Fe or Mn oxyhydroxides, which are widely considered a dominant REE host phase, are not sufficiently abundant sediment components to account for the high Nd concentrations recovered in reductive leaches, and are unlikely to be the primary source of pore water REEs. Pore water REE signatures similar to river sourced clays indicate a detrital clay dissolution source, while the spread in heavy to light REE enrichment in pore fluids and bottom waters relative to this clay source is best explained by fractionation during authigenic clay uptake of REEs. We therefore conclude that clay mineral dissolution and authigenesis are likely the primary influences on the REE cycling near the seafloor. We propose that the balance between dissolution and authigenesis controls the concentration, ratio of heavy and light REE abundances, and the isotopic composition of the pore waters. We discuss the implications of this hypothesis on an oceanic REE budget controlled by a benthic flux from a sedimentary REE source, and the use of authigenic neodymium isotopes as a paleoproxy for shifts in ocean circulation.

Harzburgite found in the Hegenshan ophiolite, southeastern Central Asian Orogenic Belt: Petrogenesis and geological implications

This paper reports detailed studies on harzburgite and serpentinite in the Hegenshan ophiolitic mélange. Harzburgite consists mainly of olivine and orthopyroxene with trace amounts of clinopyroxene and chromian spinel. Clinopyroxene occurs as isolated crystals or in the intergrowth of chromian spinel–clinopyroxene–orthopyroxene. Harzburgite is moderately to highly depleted, displaying high Fo contents in olivine (90.8–92.2), moderate Al2O3 contents in orthopyroxene (1.59–2.79 wt%), low heavy REE abundances in clinopyroxene, and moderate Cr# values of spinel (0.50–0.62). The modal proportions of olivine and orthopyroxene pseudomorph grains imply that the parent of the Hegenshan serpentinite should be harzburgite. Whole-rock compositions of the harzburgite and serpentinite samples are characterized by depletions in Al2O3 and CaO and enrichments in light REE, Sr, and U. Geochemical modeling suggests that the Hegenshan harzburgite represents residues after 17–18% partial melting of the primitive mantle. The melt in equilibrium with clinopyroxene is more depleted than typical forearc basalt and boninite. Various pyroxene thermobarometers yield equilibrated temperatures of 945–1067 °C and pressures of 4.8–8.0 kbar for the Hegenshan harzburgite. The oxygen barometer yields results of +0.4 to +1.7 log units above the fayalite–magnetite–quartz buffer for the Hegenshan harzburgite. These petrological and geochemical characteristics, as well as the estimated P–T–fO2 conditions support a back-arc setting for the Hegenshan ophiolitic mélange.

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.

Continental origin of the Gubaoquan eclogite and implications for evolution of the Beishan Orogen, Central Asian Orogenic Belt, NW China

The Gubaoquan eclogite occurs in the Paleozoic Beishan Orogen of NW China. Previously it has been interpreted as a fragment of subducted oceanic crust that was emplaced as a mélange within continental rocks. Contrary to this, we demonstrate that the Gubaoquan eclogite protolith was a Neoproterozoic basic dyke/sill which intruded into Proterozoic continental rocks. The SHRIMP UPb zircon dating of the metamorphic rims of the Gubaoquan eclogite yields an age 466±27Ma. Subdued heavy rare earth element abundances and lack of negative Eu anomalies of the metamorphic zircon domains confirm that this age represents eclogite facies metamorphism. The host augen orthogneiss has a UPb zircon age of 920±14Ma, representing the timing of crystallization of the granitic protolith. A leucogranitic vein which intrudes the eclogite has a UPb zircon age of 424±8.6Ma. This granitic vein marks the end of high-grade metamorphism in this area.The overcomplication of tectonic history of the Beishan Orogen is partially caused by inconsistent classifications and nomenclature of the same rock units and arbitrary subdivisions of Precambrian blocks as individual microcontinents. In an attempt to resolve this, we propose a simpler model that involves the partial subduction of the northern passive margin of the Dunhuang Block beneath the active continental margin developing on the Mazongshan-Hanshan Block to the north. Ocean closure and continental collision during the Late Ordovician resulted in continental thickening and eclogite facies metamorphism recorded by the mafic dykes/sills (now the Gubaoquan eclogite). In the light of the new data, the tectonothermal evolution of the Beishan Orogen is reviewed and integrated with the evolution of the Central Asian Orogenic Belt.

Petrogenesis of Triassic granitoids in the East Kunlun Orogenic Belt, northern Tibetan Plateau and their tectonic implications

The East Kunlun Orogenic Belt (EKOB), an important part of the Greater Tibetan Plateau, is an ideal region for understanding the tectonic evolution of the Anyemaqen Ocean. Here, we present zircon U–Pb ages, bulk-rock major and trace element analyses and Sr–Nd–Hf isotope compositions on representative samples of the syn-collisional Dulan batholith at the eastern end of the EKOB. The zircon U–Pb age data indicate that the bulk of the Dulan batholith was emplaced at 240–235Ma. The granitoids have high- to medium-K and metaluminous characteristics. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in some high field strength elements (HFSEs, e.g., Nb and Ta), while having a flat heavy REE (HREEs) pattern. The mafic magmatic enclaves (MMEs) share the same age, mineralogy and indistinguishable Sr–Nd–Hf isotopes with their granitoid hosts except for the higher HREE abundances. We show that the MMEs represent cumulate formed at earlier stages of the same magmatic system. The trace element data (e.g., Nb/Th, Ta/U) and inherited mantle isotopic characteristics of the Dulan batholith are also consistent with an origin via partial melting of the last fragments of underthrusting ocean crust. Simple mass balance calculations using the Sr–Nd–Hf isotopic data show that ~85% Paleo-Tethys MORB and ~15% mature crustal material (the Proterozoic gneiss of the study area) contribute to the source of the granitoids. The Dulan batholith shows compositional similarities to the bulk continental curst with inherited mantle isotopic signatures. The syn-collisional felsic magmatism must have contributed to the net continental crust growth in the EKOB. We infer that the Kunlun and Qinling orogens may actually be one single orogen offset later by the Wenquan fault system.

Geochemical, microthermometric, and isotopic constraints on the origin of fluorite deposits in central Anatolia, Turkey

We investigate rare earth element geochemistry, microthermometric characteristics, and radiogenic isotope systematics of fluorites and stable isotope compositions of gangue minerals from several fluorite deposits in central Turkey. In the deposits, fluorite is the main ore mineral and it is accompanied by quartz, calcite, and minor pyrite and barite. Veins are represented by three different fluorite types based on their color. Total REY contents of fluorites are highly variable, ranging from 24 to 693 ppm. LREE concentrations of fluorites of all colors are similar but medium and heavy REE abundances of green fluorites are nearly an order of magnitude greater than in both host rocks and purple and yellow fluorites, indicating multiple sources for crystallization. REEs show significant fractionation and purple fluorites with relatively low HREE contents were likely precipitated at an earlier stage. As crystallization continued, green fluorites were nucleated because of ion exchange of LREEs with the host rock/minerals. Fluid inclusions yielded a wide range of homogenization temperatures from 86 °C to 292 °C and salinities from 0 to 20 wt.% NaCl equiv. The 87Sr/86Sr ratios of fluorites, varying from 0.707627 to 0.709380, overlap with the range of host rocks. 143Nd/144Nd values suggest two populations: purple fluorites with less radiogenic and green fluorites with more radiogenic Nd isotope ratios. The Sr-Nd isotope systematics of Bayindir fluorites are consistent with that of the Bayindir syenite, indicating that hydrothermal solutions progressively reacted with the host rock until equilibrium was established. δ18O values of quartz are slightly higher than the magmatic range. δ13C and δ18O of calcites fall into the range of marine carbonates. δ34S values of barites indicate derivation from diverse reservoirs changing from marine to terrestrial sources. In contrast, sulfur in pyrites points to a magmatic origin. Therefore, is it suggested that magmatic fluids to some extent contributed to the precipitation of fluorite veins.

Geochemical and Hf isotopic compositions of Late Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China: petrogenesis and tectonic implications

ABSTRACTLate Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China, are located along the southern margin of the Mongol–Okhotsk orogenic belt. They comprise granodiorite, monzogranite, syenogranite, and lesser gabbro–diorite, of adakitic and calc­alkaline affinity. The adakite-like and calc­alkaline granites share similar light rare earth elements (LREE) characteristics; however, their heavy rare earth elements (HREE) trends differ from one another. The relative abundances of HREE in the calc­alkaline granites are relatively consistent and are similar to those of intrusive rocks formed from dehydration melting of garnet-free amphibolitic source rocks at relatively low pressures. In contrast, the adakite-like granites show more prominent HREE fractionation trends, indicating that they crystallized at higher pressures, where garnet in the source rocks was stable. At least two isotopically distinct sources were involved in the petrogenesis of the granites, but the extent to which they contributed varies between plutons. Most intrusions have incorporated an isotopically primitive component, possibly juvenile mafic crust. The other sources include a small proportion of old continental crustal material and isotopically evolved wall rocks. The gabbro–diorites have high MgO contents (>7 wt.%), a high Mg# (>0.6), and show moderate LREE and HREE fractionation, indicating they formed from the melting of subducted metasomatized lithospheric mantle. All of the intrusions in the study area are characterized by a relative enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicating they were emplaced in an Andean-type active continental margin setting related to southward subduction of the Mongol–Okhotsk oceanic plate.

Early-Cretaceous highly fractionated I-type granites from the northern Tengchong block, western Yunnan, SW China: Petrogenesis and tectonic implications

Western Yunnan, an important constituent of the southeastern segment of the East Tethyan tectonic domain, lies along a transformed orientation from the NWW trending Himalayan–Tethyan segment to the northerly trending Southeast Asian segment. However, the geodynamical setting of the Early Cretaceous tectonothermal magmatism along the Bangong–Nujiang–Lushui–Luxi–Ruili belt as the Tethyan branch in western Yunnan (SW China) remains controversial. The Donghe granitoid, which is located between the Gaoligong and Tengliang belts in the northern Tengchong block, reveals its petrogenesis and its tectonics, both of which play a vital role in resolving previous disputes. Our zircon laser ablation inductively coupled plasma mass spectrometry U–Pb dating of granites from the Donghe batholith yields ages of 119.9±0.9–130.6±2.5Ma. These granites display features typical of highly fractionated I-type granites: high SiO2 contents (>71wt.%), high K contents (K2O=3.88–5.66wt.%), calc-alkaline character, slight peraluminosity (A/CNK=1.02–1.16), and a highly differentiated index ranging from 83.6 to 95.6. In addition, as SiO2 contents increase, the rare earth element (REE) abundances, especially heavy REE abundances, and REE pattern slopes change gradually, but the negative Eu anomalies increase sharply, while the degree of enrichment in Rb, Th, U, and Pb and depletion in Ba, Nb, Sr, P, and Ti are enhanced. These features indicate that K-feldspar, ±plagioclase, ±biotite, ±amphibole, ±apatite, ±sphene/garnet, and ±Fe–Ti oxides such as ilmenite play the major role in the fractional crystallization process. The high initial 87Sr/86Sr (0.7067 and 0.7079) and negative εNd(t) values (−8.6 to −10.1), with T2DM ranging from 1.39 to 1.49Ga, indicate that the sources were mainly derived from the mature ancient middle to lower crust and minor mantle-derived materials. The initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.462–18.646, 15.717–15.735, and 38.699–39.007, respectively, signify that some subduction-related material such as ocean island volcanic rocks and mature arc primitive rocks may be involved as sources. Based on an analysis of similar zircon saturated temperature and geochemical characteristics of typical highly fractionated I-type granites in SE and SW China, and consideration of the regional geological setting, we suggest that the parent magma may be derived from the ancient middle to lower continental crust and mantle-derived basaltic magma. These were generated in the setting of a westward subducted Lushui–Luxi–Ruili (LLR) Tethyan oceanic slab, where mantle-wedge-derived sources provided enough heat and material to melt the middle to lower ancient crust. Taking into account the temporal–spatial distributions of Early Cretaceous magmatic rocks in the region, we further suggest the existence of an Andean-type active continental margin from the Lhasa block to the Tengchong block along the Bangong–Nujiang–LLR Tethys Ocean during the Early Cretaceous.

An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites

The distribution of rare earth elements (REEs) in mantle minerals depends on temperature, pressure, and mineral composition. We developed parameterized lattice strain models for rare earth element partitioning among mantle minerals (olivine, orthopyroxene, clinopyroxene, and garnet) using mineral–melt partitioning models that were calibrated against experimentally determined high quality mineral–melt REE partition coefficients. The validity of the mineral–mineral REE partitioning models were independently tested using measured REE abundances in minerals from well-equilibrated mantle peridotites and eclogites reported in the literature. Using the new partitioning models, we critically assessed the extent of subsolidus re-equilibration on REE distribution among major rock-forming minerals in spinel and garnet peridotites. During subsolidus re-equilibration, REEs are preferentially partitioned into clinopyroxene in peridotites. The extent of REE redistribution in clinopyroxene depends on the modal abundance of clinopyroxene in the peridotite. Subsolidus exchange with orthopyroxene and olivine can increase heavy REE abundances in clinopyroxene from spinel harzburgite and middle REE abundances in clinopyroxene from garnet harzburgite by a factor of three. The higher than expected heavy REE abundances in clinopyroxene from spinel harzburgite may give rise to a false garnet signature in residual peridotites. The extents of melting for abyssal peridotites based on REE abundances in clinopyroxene can be significantly over-estimated without proper correction for subsolidus re-equilibration. REE abundances in both clinopyroxene and orthopyroxene are needed to assess the melting processes from residual peridotites.

Geochemical constraints on the petrogenesis of granitoids in the East Kunlun Orogenic belt, northern Tibetan Plateau: Implications for continental crust growth through syn-collisional felsic magmatism

Early Triassic syn-collisional granitoids with mafic magmatic enclaves (MMEs) crop out along the entire East Kunlun Orogenic belt (EKOB) at the northern margin of the Tibetan Plateau. They are andesitic in composition and enriched in light rare earth elements (LREEs) with a flat heavy REE (HREE) pattern. Their average composition resembles that of the bulk continental crust. The enclosed MMEs have the same mineralogy as their host granitoids, but contain a greater mode of mafic minerals (amphibole and biotite), and thus have higher HREE abundances. Zircon U–Pb dating shows that both the granitoid hosts and MMEs have the same crystallization age of ~250Ma and indistinguishable bulk rock Sr–Nd–Pb–Hf isotope compositions (ISr of 0.7080–0.7116, varying 206Pb/204Pbi of 18.53–19.32, essentially constant εNd(t) of −5.3 to −2.1 and a small range of positive εHf(t), mostly 1.7–5.2). The complete isotopic overlapping between the granitoid hosts and the MMEs is understood to reflect that the MMEs are disintegrated cumulates formed at an early stage of the granitoid magma evolution within the same magmatic system. The isotopic data set reveals that the granitoids are variably evolved melts produced by partial melting of the subducted Paleo-Tethyan oceanic crust with terrigenous sediments under amphibolite-facies conditions in response to the continental collision.

통가열도 해저화산 표층 퇴적물 내 열수변질의 화학적 특성

통가열도 내 우리나라 해저열수광상 독점 탐사지역에 위치하는 5개 해저화산(TA12, TA19, TA22, TA25, TA26) 주변 표층 퇴적물의 열수변질 여부를 규명하고, 열수변질 특성을 확인하기 위하여 각 해저화산에서 채취된 29개의 표층 퇴적물에 대해 미량 원소와 희토류 원소 분석을 실시하였다. 분석 결과 TA12, TA19, TA22 해저화산에서는 열수에 의한 퇴적물의 변질작용이 없거나 매우 적은 것으로 나타났으며, TA25와 TA26 해저화산의 일부 지역은 현재에도 열수의 영향을 받고 있는 것으로 나타났다. 열수의 영향이 있는 해저화산 사이에도 부화된 미량 원소의 종류에 차이가 있는데, TA25 해저화산의 경우 주로 Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, As가, TA26 해저화산에서는 주로 Cu, Sn, Zn, Pb, Cd, Sb, Ba, Rb, Sr이 부화되어 있다. 미량 원소의 부화가 확인된 지역은 희토류 원소의 부화도 관찰된다. 상부지각 희토류 원소 농도로 표준화된 희토류 원소의 분포형태는 가벼운 희토류 원소(LREE)가 매우 낮고 중간 희토류 원소(MREE)에서 무거운 희토류 원소(HREE)로 갈수록 증가하는 형태를 보이며, TA25와 TA26 해저화산의 일부 정점에서 Eu의 부화가 확인된다. 또한 TA26 해저화산의 일부 정점에서 Ce이 부화가 관찰되었는데, 이는 주변지역에서 채취된 열수 내 희토류 원소의 높은 Ce분포형태와 매우 유사하다. 또한 TA25와 TA26 해저화산은 부화된 원소의 특징이 다르게 나타나는데, TA25 해저화산에서는 Cu를 포함한 미량 원소가, TA26 해저화산에서는 Ce와 Eu를 포함한 희토류 원소가 우세하게 부화되어 있는 것으로 나타났다. 조사결과 나타난 각 해저화산 표층 퇴적물 내 부화된 미량 원소 및 희토류 원소의 종류와 농도는 차후 통가열도 지역의 열수광상 탐사시에 지역적인 열수변질의 지시자로써 유용하게 활용될 것으로 판단된다. We analyzed 29 surface sediment samples in five submarine volcanoes (TA12, TA19, TA22, TA25, and TA26) located in the southern part of the Tonga arc for trace elements and rare earth elements to investigate characteristics of the hydrothermal alteration of surface sediments. Based on analytical results of trace element and rare earth element (REE), surface sediments of TA12, TA19, and TA22 submarine volcanoes, which are located in the northern part of the study area, were very little or not influenced by hydrothermal fluids. In contrast, some stations of TA25 and TA26 submarine volcanoes were strongly affected by hydrothermal fluids. However, these two submarine volcanoes showed different features in element concentration in the sediments. Some stations of TA25 submarine volcano showed enrichment of Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, and As, however, those of TA26 submarine volcano showed enrichment of Sn, Zn, Pb, Cd, Sb, Ba, Rb, and Sr. Stations which enriched trace elements were observed, enriched REEs were also observed. Average upper continental crust (UCC)-normalized REE patterns of the surface sediments generally showed low light REE (LREE) abundances and increased heavy REE (HREE) abundances. Eu enrichment was identified at several stations of TA25 and TA26 submarine volcanoes. In addition, enrichment of Ce was found at some stations of TA26 submarine volcano and these enrichment patterns were similar with hydrothermal fluid of near stations. Furthermore, TA25 and TA26 submarine volcanoes showed different enrichment characteristics of trace elements and REE. Trace elements were concentrated at TA25 submarine volcano. TA26 submarine volcano, on the other hand, observed highly enrichment of REE especially, Eu and Ce. As a result of the investigation, the characteristics and concentrations of REEs and trace elements in the surface sediments of each submarine volcano can be applied to identify hydrothermal alteration of sediments during exploration for hydrothermal deposits.

Synchronous Oligocene-Miocene metamorphism of the Pamir and the north Himalaya driven by plate-scale dynamics

Gneiss domes in the Pamir (Central Asia) and the Himalaya provide key data on mid- to deep-crustal processes operating during the India-Asia collision. Laser ablation split-stream inductively coupled plasma–mass spectrometry (LASS-ICP-MS) data from monazite in these domes yield a time record from U/Th-Pb dates and a petrologic record from rare earth element (REE) abundances. Seven samples from the Pamir and six samples from the north Himalayan gneiss domes yield almost identical monazite dates of ca. 28–15 Ma. Most monazite has invariant heavy REE (HREE) abundances; two samples, however, have older monazite that records progressive HREE depletion and two samples have younger monazite that records progressive HREE enrichment. These variations in HREE are compatible with increasing garnet abundance—prograde metamorphism— until ca. 20 Ma, and decreasing garnet abundance thereafter. The change from HREE depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling. This documentation of synchronous Barrovian metamorphism within domes of Indian crust along the margin of the orogen (Himalaya) and within domes of Asian crust within the core of the orogen (Pamir) is best explained by a plate-scale driving force rather than by local events. We propose that widespread, synchronous thickening was initiated by the resumption of Indian subduction following slab breakoff and then terminated by a second slab-tearing event—both plate-scale events inferred from tomography.

The generation of high Sr/Y plutons following Late Jurassic arc–arc collision, Blue Mountains province, NE Oregon

High Sr/Y plutons (Sr/Y > 40) occupy large areas in ancient and modern orogenic belts, yet considerable controversy exists regarding mechanisms of their generation, the tectonic settings in which they form, and their relationship to contractional deformation through time. In the Blue Mountains province (NE Oregon), a suite of Late Jurassic (148–145 Ma), high Sr/Y plutons intrude Middle Jurassic (162–157 Ma), low Sr/Y (< 40) arc-related lavas and plutons in the Dixie Butte area immediately after widespread Late Jurassic arc–arc collision (159–154 Ma). Early, pre- to syn-kinematic low Sr/Y lavas and plutons (162–157 Ma) have flat to slightly enriched light rare earth element (REE) abundances, low Sr (< 400 ppm) and Sr/Y values (< 40), and strongly positive initial epsilon Hf values (+ 10.1 to + 12.3: 2σ weighted average). Ce/Y values from basalts and gabbros yield a maximum crustal thickness of ~ 23 km. These geochemical and isotopic features suggest derivation from a depleted-mantle source and/or shallow-level (≪ 40 km) melting of pre-existing island arc crust with little to no evolved crustal input. In contrast, post-kinematic high Sr/Y plutons (148–145 Ma) are more compositionally restricted (tonalite–trondhjemite–granodiorite) and display depleted heavy REE abundances, an absence of Eu anomalies, elevated Sr (> 600 ppm) and Sr/Y values (> 40), and positive initial epsilon Hf values (+ 10.5 to + 7.8: 2σ weighted average). These geochemical and isotopic results are consistent with geochemical models suggesting derivation from partial melting of island arc crust in the presence of a plagioclase-poor to absent, clinopyroxene + hornblende + garnet-bearing source (depths > 35–40 km). We propose that the transition from low Sr/Y to high Sr/Y magmatism resulted from orogenic thickening of island arc crust in the Dixie Butte area during Late Jurassic arc–arc collision between the Olds Ferry and Wallowa island arcs at 159–154 Ma. This fundamental change in crustal structure influenced post-orogenic magmatism and resulted in a relatively brief (~ 3 myr: 148–145 Ma) episode of high Sr/Y magmatism. Other high Sr/Y plutons occur throughout the US sector of the western North American Cordillera (e.g., Salmon River suture zone, Klamath Mountains, Peninsular Ranges) and closely follow major contractional events involving arc–arc and arc–continent collisions.

A unique Mo deposit associated with carbonatites in the Qinling orogenic belt, central China

The Qinling molybdenum belt is a prominent metallogenic structure in central China hosting several significant porphyry- and porphyry–skarn-type deposits. The Huanglongpu Mo deposit in the north-western part of the belt is unique in that it is associated with carbonatite dykes, rather than felsic magmatism. The carbonatites are composed largely of Sr–Mn-rich calcite and characterized by high concentrations of Sr and rare-earth elements (REE), and stable-isotope values indicative of a mantle source ( δ 13C PDB = − 6.7 ± 0.2‰ and δ 18O SMOW = 8.2 ± 1.0‰). Molybdenite is associated with galena and REE minerals (parisite, bastnäsite and monazite). Both molybdenite and galena are characterized by high Re contents (up to 0.4 and 0.2 wt.%, respectively) and Re/(Mo, Pb) ratios approaching the primitive-mantle values. In contrast to the rock-forming calcite, the REE minerals are enriched in light REE, whose relative proportion increases from parisite-(Ce) [average (La/Nd) n = 2.1] to bastnäsite-(Ce) and monazite-(Ce) [average (La/Nd) n = 3.1, 4.6, respectively]. The whole-rock compositions are characterized by some of the highest Mo and heavy REE abundances reported for carbonatites to date: up to 1010 ppm Mo, 1130 ppm Y + Gd…Lu and (La/Yb) n = 1.2–2.7. The unusual trace-element geochemistry of the Huanglongpu rocks may ultimately reflect the composition of their mantle source, but their enrichment in Mo + Re was undoubtedly enhanced through preferential partitioning of these elements into a light REE–Pb–S-rich fluid derived from the carbonatitic magma modified by calcite fractionation. The present work shows that Mo can be retained, transported and deposited by carbonatitic fluids capable of generating economic Mo deposits.

Chemical composition of whole-rocks samples and volcanic glasses of the Miocene Tamateyama and Sekibutsu Tuffs in the area surrounding the Nara Basin

We used XRF and INAA methods to analyze the chemical composition of whole-rock and volcanic glass samples from the Tamateyama and Sekibutsu Tuffs in the area surrounding the Nara Basin. Both tuffs have rhyolitic compositions with SiO2=73.6-75.0 wt.%, similar to that of the Muro Pyroclastic Flow Deposits (Muro PFD). The REE patterns of the tuffs are characterized by light REE enrichment, a pronounced negative Eu anomaly, and relatively constant chondrite-normalized middle to heavy REE abundances. These features are similar to those of S-type felsic rocks of the Outer Zone of southwest Japan. Our new petrochemical data provide additional constraints on correlations of the Tamateyama Tuff, Sekibutsu Tuff, and Muro PFD, and indicate that these units were derived from a large felsic igneous body in the Outer Zone of the Kii Peninsula.

Origin of cratonic lithospheric mantle roots: A geochemical study of peridotites from the North Atlantic Craton, West Greenland

A critical examination of the extent to which geodynamic information on the initial mantle depletion and accretion event(s) is preserved in kimberlite-borne cratonic SCLM peridotite xenoliths is attempted by using new major and trace element data of whole-rock peridotites ( n = 55) sampled across the North Atlantic Craton (NAC; West Greenland). We also present additional whole-rock trace element data of mantle xenoliths from Somerset Island, the Slave and Kaapvaal cratons for comparison. Peridotites comprising the West Greenland SCLM are distinctly more olivine-rich and orthopyroxene-poor than most other cratonic peridotites, in particular those from the Kaapvaal craton. The West Greenland peridotites have higher Mg/Si but lower Al/Si, Al 2O 3 and CaO than cratonic mantle from the Kaapvaal Craton. We suggest that the more orthopyroxene depleted, harzburgite to dunite character of the NAC peridotites reflects more of the original melting history than peridotites from other cratons and in that sense may be more typical of cratonic lithosphere compositions prior to extensive modification. Despite this, some modal and cryptic metasomatism has clearly taken place in the West Greenland lithosphere. The insensitivity of major elements to pressure of melting at high degrees of melt extraction combined with the ease with which these elements may be changed by modal metasomatism mean that we cannot confidently constrain the depth of melting of peridotites using this approach. Mildly incompatible trace elements offer much more promise in terms of providing geodynamic information about the original Archean melting regime. The very low, systematically varying heavy REE abundances in NAC whole-rock peridotites and in peridotites from all other cratons where high-quality data are available provide ubiquitous evidence for a shallow melting regime in the absence of, or to the exhaustion of garnet. This finding explicitly excludes large extents of deep (iso- and polybaric) melting, which results in high initial garnet abundances and increasing heavy REE abundances. This evidence renders models that invoke large plume-like melting environments redundant in explaining SCLM formation and suggests broadly modern plate tectonic environments are responsible for the depletion of cratonic SCLM. A combination of the shallow melting environment and uniformly high levels of depletion indicate that melting to form the NAC lithosphere and that of other cratons probably took place in a subduction-zone environment.

Late Neoproterozoic carbonate productivity in a rifting context: the Adoudou Formation and its associated bimodal volcanism onlapping the western Saghro inlier, Morocco

AbstractAn interval of episodic carbonate productivity, lithostratigraphically recognized as the ‘Calcaires inférieurs’ (upper member of the Adoudou Formation), took place across the Neoproterozoic–Cambrian transition onlapping the western Saghro inlier, Morocco. Sedimentation of the ‘Calcaires inférieurs’ was highly variable: in relatively stable substrates, a peritidal-dominated mixed platform is recorded where deposition was primarily controlled by autocyclic processes and accommodation space availability, whereas, in unstable substrates, the tectonic activity associated with the inherited block-faulting basement led to deposition of complex slide sheets composed of penecontemporaneous isoclinal folds and disrupted strata. The uppermost part of the ‘Calcaires inférieurs’ displays a negative δ13C shift reaching values of −6.5‰. This shift may represent the δ13C excursion to −6‰ that marks the Neoproterozoic–Cambrian boundary in the western Anti-Atlas. Two volcanic episodes bracketed the carbonate productivity. They consist of lower basaltic flows and an upper rhyolitic ignimbrite, with a SiO2 gap between 52 and 74 wt%. The basic rocks resemble those of tholeiitic magmas in continental rifts. The felsic rocks show high light to heavy rare earth element abundances and negative Nb, Ta, P and Ti anomalies, and were probably generated as a result of either fractional crystallization coupled with relative crustal contamination, or from a different magmatic source. The lower basic flows of tholeiitic affinity predated and geochemically differ from the alkaline magmatism of the Alougoum volcanic complex (Boho jbel) that surrounds the neighbouring Bou-Azzer inlier.