Enriched Mantle Research Articles

Sr isotopes and geochemistry of Cerro Overo maar provide a unique window into arc magma genesis in the Central Volcanic Zone of the Andes

Cerro Overo maar is the most mafic lava (54 wt% SiO2, 7.4 wt% MgO) found within the modern subduction arc of the Central Andes. Structural weaknesses in the crust allowed magma to rapidly ascend while avoiding extensive crustal contamination, interactions with the mid-crustal Altiplano-Puna Magma Body, or storage in magma chambers. Cerro Overo lava thus preserves chemical and mineralogical signals from deep-crustal or upper-mantle magmatism which are otherwise absent in the modern Andes Central Volcanic zone (CVZ). Radiogenic isotope ratios of CVZ rocks are characteristically enriched, and researchers have debated for decades whether this enrichment is derived from an enriched mantle source or the result of crustal processes. Phenocrysts of high-forsterite olivine (Fo86) from Cerro Overo contain three populations of silicate melt inclusions: 1. basaltic andesite inclusions chemically identical to the host lava, 2. rhyodacite inclusions representing assimilated upper crust, and 3. basaltic melt inclusions (SiO2 = 49.0 wt%) of minimally-altered magma, reflecting an early-stage melt which may be common to the active arc of the Altiplano-Puna region. Melt inclusion 87Sr/86Sr (0.70376 to 0.70720) also form three clusters, including the most primitive Sr isotopic ratios reported for the modern CVZ arc: direct evidence that the sub-arc mantle source retains non-radiogenic Sr signatures. The least-evolved Olivine-hosted melt inclusions from Cerro Overo match the composition of postulated arc endmembers and provide direct evidence that the overall radiogenic isotopic characteristic of the modern CVZ results from lithospheric processes and not inheritance from a mantle source with an initially radiogenic 87Sr/86Sr signature.

Mesozoic mafic dykes in the North China Craton: magmatic evolution and implications for gold mineralization

ABSTRACT The North China Craton (NCC), which underwent multiple subduction and lithospheric destruction during the Mesozoic, is well known for hosting some of the world-class gold deposits. In many cases, Mesozoic mafic dykes are associated with the gold deposits, although there has been no systematic studies aimed to integrate the characteristics of these dykes in different regions of the NCC. Here we investigate diabase and lamprophyre dyke suites in the Xiaoqinling region along the southern margin of the NCC, and compile the published geochronological, geochemical and isotopic data for regional mafic dykes and gold deposits from the major gold belts including Jiaodong, Xiaoqinling and central NCC. Zircon U-Pb data show the mafic dykes in Xiaoqinling were emplaced in two episodes at 219–227 Ma and 121–137 Ma, and their εHf(t) values (ranging from −2.1 to 3.8 and −13.6 to 1.6 respectively) suggest increasing input of lithospheric materials in the Early Cretaceous dykes. The compiled geochemical data on the mafic dykes from different regions in the NCC show that the magmatic evolution involved olivine and clinopyroxene fractional crystallization with limited crustal contamination and subduction-related fluid metasomatism. The geochemical and isotopic signature suggests multiple sources for the parent magma of the mafic dykes ranging from convective asthenospheric mantle to ancient lithospheric mantle, with the magma source characterized by enriched mantle (EM2 or EM2+ EM1), and that signatures might be potential for gold mineralization. Further, most dykes are coeval and cospatial with gold deposits in the NCC. The geochronological data show that small amount of both Mesozoic dykes and gold deposits formed in Triassic whereas the peak formed in Cretaceous following slab roll back of the Pacific plate subducting from the east during 145–110 Ma. We envisage that the tectonic extension associated with Pacific slab subduction contributed to the magmatism and gold mineralization in the NCC.

Oblique continental collision and the formation of syn-collisional A-type granites: insights from the Early Jurassic Baoji granite suite in the Qinling orogenic belt, central China

ABSTRACT A-type granites are conventionally interpreted to be formed in late post-collisional or anorogenic settings. However, we firstly identified an Early Jurassic syn-collisional A-type granite suite in the Qinling orogenic belt: the Baoji granite suite. A comprehensive investigation into its mineralogical, petrological, geochronological, and geochemical characteristics can shed light on the genesis of syn-collisional A-type granites and geodynamic evolution of collisional orogeny. The Baoji granite suite contains two types of rocks: (1) A-type granites including biotite syenite, coarse-grained, porphyritic, and fine-grained syenogranites, and alkali-feldspar granite, and (2) magnesian monzodiorite; both types are coeval, with zircon U–Pb ages of ca. 190 Ma. The A-type granites yield A1-type trace element signatures and large variations in trace element contents, with Ba contents of 16.9–762 ppm, Sr of 11.0–211 ppm, Rb of 193–645 ppm, and Eu of 0.08–1.24 ppm. Their radiogenic isotope compositions (87Sr/86Sri = 0.70631–0.70637; εNd = −5.18 to −3.84; εHf = −5.01 to +2.76) are similar, and overlap those of Neoproterozoic OIB-like basic volcanics from the South Qinling belt. These characteristics show that the A-type granites originated from the anatexis of OIB-like lower crust and underwent fractional crystallization in a silicic magma reservoir. The monzodiorite has low SiO2 contents (49.82–53.29 wt.%) and enriched radiogenic isotopic compositions (87Sr/86Sri = 0.70554–0.70638; εNd = −11.0 to −5.28) contains abundant hornblende and biotite (~40%), and exhibits evidence of magma mingling. It also contains two groups of zircon: pristine and unzoned zircon that yields negative Hf isotope compositions (εHf = −9.42 to −3.14), and xenocrystic zircon that yields similar Hf isotope compositions (εHf = −3.54 to +1.85) to the A-type granites. These characteristics demonstrate that the monzodiorite was derived from the differentiation of basaltic partial melts of an enriched mantle source, and mingled with the A-type granitic magma during its ascent and emplacement into a silicic magma reservoir. Along with regional records of the Early Jurassic magmatism, deformation, metamorphism, and sedimentation, the genetic models for the Baoji A-type granites and monzodiorite suggest that the Middle Triassic to Early Jurassic oblique continental collision (ca. 230–190 Ma) produced coupled EW-trending strike-slip and NE-trending extensional faults at crustal levels and Rayleigh–Taylor (gravitational) convective instabilities at mantle depths in the Qinling orogenic belt, which controlled the production of the syn-collisional A-type granite suite. Our work also indicates that the A-type granites can be formed in syn-collisional setting in response to oblique continental collision and the resultant slow foundering of a high-density lithospheric root; therefore, we suggest that the syn-collisional A-type granites are evidence for oblique continental collision.

Leucite basanites of Virunga (East African Rift): some insights into petrogenesis and source composition

The leucite basanites of the Visoke and Muhavura volcanoes from the province of Virunga (Western Branch of the East African Rift) were investigated in detail. The major and trace element contents and Sr, Nd and Pb isotopic composition in the whole rocks were determined. The isotope ratios (87Sr/86Sr: 0.70644–0.70793; 143Nd/144Nd: 0.51233–0.51243) show that these rocks are far from the bulk Earth on the “mantle trend” and enriched in radiogenic Sr. The major element compositions reveal that they are not primary mantle melts as they have Mg# = 0.50–0.59. Moreover the leucite basanites are enriched by LILE and LREE relative to PM which may indicate an enriched mantle source. Pb isotopes of the studied leucite basanites (206Pb/204Pb: 19.112–19.438, 207Pb/204Pb: 15.737–15.783, 208Pb/204Pb: 40.509–40.866) along with the data of Karisimbi volcano form trends on isotope diagrams towards the EM II or ancient continental crust. The position of leucite basanite data on the 87Sr/86Sr - 208Pb*/206Pb* correlation line suggests that they can be a result of melt enrichment/vein metasomatism within the mantle.The phenocryst composition of leucite basanites varies in range: olivine- from Fo55 to Fo89 with NiO content 0.01–0.34% wt.; clinopyroxene - from 0.65 up to 0.86 Mg# with Cr2O3 from 0.02 to 1.5% wt., plagioclase corresponds to labradorite (50–70% An). The ore minerals present are titanomagnetite and ilmenite. The evaluation of T-fO2 based on titanomagnetite and ilmenite composition shows that phenocryst crystallization took place under QFM buffer. The presence of quartz and rutile xenocrysts in one Visoke leucite basanite lava points to the contamination of this melt by crustal material. At the same time, nonequilibrium megacrysts of high-magnesian olivine (Fo89) from another sample of Visoke leucite basanite resulted from leucite-basanite magma mixing with a more primitive melt.We identify melts from two different sources at Visoke edifice: ugandite and its possible derivatives were formed from the mantle source, whereas the leucite basanites are enriched in radiogenic strontium and could be contaminated by deep metamorphic rocks. The origin of the studied leucite basanites is the result of a combination of various processes: melting of the metasomatized mantle, fractional crystallization, magma mixing and crustal contamination.

Early Neoproterozoic crustal growth and microcontinent formation of the north–central Central Asian Orogenic Belt: New geological, geochronological, and Nd–Hf isotopic data on the Mélange Zone within the Zavkhan terrane, western Mongolia

In this study, new geological, geochronological, geochemical, and Nd–Hf isotopic data are presented for the Mélange Zone within the Zavkhan terrane, Mongolia, and the terrane structure, early Neoproterozoic continental crust growth, and microcontinent formation in the north–central part of the Central Asian Orogenic Belt (CAOB) are discussed. The Mélange Zone separates high-grade complexes of the northwestern part of the Zavkhan terrane and unmetamorphosed Neoproterozoic Zavkhan Formation covered by Cryogenian–Cambrian shelf deposits of the southwestern part. Zone consist of a lower-grade association of basalts, basaltic andesites, rarely felsic volcanic rocks, trondhjemites of the Kharuul Massif and variably metamorphosed from greenschist- to upper amphibolite-facies, and high-grade metamorphic rocks including quartzite–gneisses, hornblende schists, and amphibolites with relics of eclogite and blueschist-facies metamorphism assemblages. Emplacement of trondhjemites and gabbro dykes of the Kharuul Massif occurred at about 960–930 Ma. Geochemical, Nd whole-rock, and Hf-in-zircon isotopic data indicate an oceanic island arc setting for the lower-grade association. LA-ICP-MS dating and Hf-in-zircon data for detrital zircons from the quartzite–gneisses of the Mélange Zone indicate that the sources comprise Palaeoproterozoic (ca. 2.02 and 2.48 Ga), Neoarchean (ca. 2.59 and 2.67 Ga), and Mesoarchean (ca. 2.8 and 3.0 Ga) magmatic and metamorphic crustal rocks as well as 2.6–2.5 Ga juvenile complexes. High-grade amphibolites show geochemical similarity to continental arc and within-plate basalts, and were formed from enriched mantle sources. Obtained combined with published data demonstrate that the Zavkhan terrane represents composite terrane composed of ca. 960–930 Ma island arc complex, and ca. 860–800 Ma active continental margin complexes in the north and reworked at ca. 800–720 Ma early Precambrian continental crust in the south, which are separated by a Mélange Zone. We speculate that formation of the latest Mesoproterozoic–early Neoproterozoic island arc and active continental margin complexes in the north–central CAOB could be related to the assembly of the Rodinia supercontinent at ca. 1.1–1.0 Ga, which initiated subduction in Mirovian Ocean and led to the development of accretionary orogens around supercontinent margins.

Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China

The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO2 contents (1.47% –3.59%) and low MgO (3.52% –7.88%), K2O (0.12% –1.21%). Large ionic lithophile elements (LILEs) (Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO2 ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of 87Sr/86Sr (ISr) (0.7052–0.7097) and εNd(t) (–0.70 to –5.35), while the Pb isotopic compositions with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.908–17.982, 15.487–15.721, 37.276–38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.©2023 China Geology Editorial Office.

Extreme Heterogeneity in Mid‐Ocean Ridge Mantle Revealed in Lavas From the 8°20′N Near‐Axis Seamount Chain

AbstractLavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on <1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid‐ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid‐ocean ridge (MOR) mantle. Normal mid‐ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid‐ocean ridge basalts (EMORB) range from <1% melting of 10% enriched mantle to >15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales.

Geochemistry, geochronology and its geological implication of metamorphic rocks of the Jiehekou Group in the Lüliang complex

华北中部造山带是华北克拉通内部一条重要的前寒武纪碰撞造山带，该造山带的形成经历了一个漫长而复杂的演化过程。吕梁变质杂岩位于造山带中段，阜平杂岩西南，是透视华北克拉通早前寒武纪构造-热演化的一个重要窗口。本文针对该地区的界河口（岩）群黑云斜长片麻岩和含榴斜长角闪岩开展了系统的野外地质调查、显微岩石学、地球化学和变质年代学研究。结果表明，界河口（岩）群黑云斜长片麻岩的原岩多为富铝富钾的变泥质岩石，其物源主要来自长英质火成岩，形成于有演化岛弧发育的活动大陆边缘构造背景；含榴斜长角闪岩的原岩多数是拉斑玄武岩，源区来自于富集地幔，形成于岛弧构造环境。显微岩相学研究显示，含榴夕线黑云斜长片麻岩和含榴斜长角闪岩中均保留了早期进变质、峰期变质和晚期退变质三个阶段的变质矿物组合，且石榴子石边部多发育明显的白眼圈反应结构。其中，含榴夕线黑云斜长片麻岩的峰期矿物组合包括石榴子石、黑云母、夕线石、斜长石、石英和少量钾长石等；含榴斜长角闪岩的峰期矿物组合主要由石榴子石、角闪石、斜长石、磁铁矿和石英等组成。前期研究表明，二者的变质演化均记录了类似的顺时针近等温降压型的变质作用P-T轨迹，其峰期变质条件分别为>750℃/>7.0kbar和>750℃/>6.3kbar。本文LA-ICP-MS锆石U-Pb年代学研究表明，含榴黑云斜长片麻岩中记录了1928~1920Ma和1882~1855Ma两组变质年龄。结合中部带前人研究成果，本文推断吕梁变质杂岩的区域变质事件与华北克拉通东、西部陆块间的碰撞造山作用有关，该地区与碰撞阶段同期的峰期变质作用发生在1928~1920Ma，而后于1882~1855Ma期间进入与快速构造隆升有关的退变质作用阶段。;As an important Precambrian orogenic belt in the North China Craton (NCC), the Trans-North China Orogen (TNCO) was formed by the collision between the Eastern Block and Western Block through an extremely protracted evolution process. To the southwest of the Fuping complex in the central TNCO, the Lüliang metamorphic complex provides an ideal target for studying the Precambrian tectonothermal evolution of the NCC since many upper amphibolite- to granulite-facies metamorphic rocks of the Jiehekou Group were exposed in this area. Petrographic, geochemical and metamorphic geochronological studies were carried out on the biotite plagiogneisses and garnet-bearing amphibolites in this contribution. It was indicated that the protoliths of biotite plagiogneisses were mostly metapelites with high aluminium and potassium contents whose sedimentary provenances were demonstrated to have mainly come from felsic igneous rocks formed in an active continental margin environment. The protoliths of garnet-bearing amphibolites were tholeiite basalts from the enriched mantle, and formed in the island arc tectonic setting. Microscopic petrographic studies showed that there were three stages of mineral assemblages preserved in the garnet-bearing sillimanite-biotite plagiogneisses and amphibolites, including the prograde, peak and retrograde stages. The white-eye socket reaction microstructures commonly grew surrounding the garnet rims of these rocks. The peak assemblages of garnet-bearing sillimanite-biotite plagiogneisses mainly consist of garnet, biotite, sillimanite, plagioclase, quartz and some K-feldspar. Those of garnet-bearing amphibolites include garnet, amphibole, plagioclase, magnetite and quartz. Our previous studies showed that both two types of rocks revealed clockwise P-T paths ended with a near-isothermal decompression, and their peak conditions were > 750℃/>7.0kbar and >750℃/>6.3kbar, respectively. LA-ICP-MS zircon U-Pb dating in this study indicated that two groups of metamorphic ages, including 1928~1920Ma and 1882~1855Ma, were recorded in the garnet-bearing biotite plagiogneisses. Combined with previous metamorphic and geochronological studies of other complexes in the TNCO, the regional metamorphism occurred in the Lüliang complex was thought to be related to the orogenic collision between the Eastern Block and Western Block of the NCC. During this orogenic process, the metamorphic peak occurred at 1928~1920Ma in the Lüliang area, and then followed with 1882~1855Ma retrograde stages resulted by the rapid tectonic uplift after collision.

Geochemical characteristics of magnesiocarbonatites from Muostalaah apatite occurrence and Birikeen apatite deposit (Aldan-Stanovoy shield, South Yakutia, Russia)

Apatite-bearing ore occurrence Muostalaah and phosphate deposit Birikeen are magnesiocarbonatite complexes of the Seligdar type, located within the Nimnyrskaya submeridional metallogenic domain in the Aldan-Stanovoy shield in South Yakutia, Russia. This study presents geochemical characteristics of carbonatites and adjacent apatite-bearing silicate rocks of the Birikeen apatite deposit and Muostalaah apatite occurrence. Comparison of these complexes with the Seligdar deposit and other magnesiocarbonatite complexes of the world was made. Based on the chemical analysis of the petrogenic components of the Muostalaah and Biriken carbonatites, it was concluded that these rocks belong to magnesiocarbonatites, similar to the magnesiocarbonatites of the Seligdar deposit. The chondrite-normalized REE spectra of magnesiocarbonatites and apatite rocks are characterized by LREE enrichment relative to HREE with average La/Yb(cn) ratios of 45 and 100 for the Muostalaah and Birikeen rocks, respectively. Most of the REE patterns are within the field of the world magnesiocarbonatites. The trace-elements abundances of carbonatites and apatite rocks record negative Nb, Pb, Sr, Zr, Hf and Ti anomalies and Th, U and REE enrichment. Сoncentrations of Th, U, Zr, and Hf are characterized by a wide spread of values. The trace-element patterns of the Birikeen and Muostalaah carbonatites are generally similar to the patterns of the world magnesiocarbonatites, including Seligdar rocks. Based on the results of isotopic studies of the Sm/Nd and Rb/Sr systems, it was concluded that the primary melts for the Muostalah and Birikeen carbonatites were formed from a long-lived enriched mantle source that separated from the depleted mantle in the Late Archean.

A major change in magma sources in late Mesozoic active margin of the circum‐Sea of Japan domain: Geochemical constraints from late Paleozoic to Paleogene mafic dykes in the Sergeevka belt, southern Primorye, Russia

AbstractMore than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major‐ and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow‐dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high‐Mg and high‐Al gabbro‐dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)‐type mantle, whereas the younger suite from an enriched mantle II (EM2)‐type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic‐earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long‐continuing subduction‐related magmatism. The plume influence reached the maximum when the unique meimechite‐picrite complex formed in the region.

Timing of Seafloor Spreading Cessation at the Macquarie Ridge Complex (SW Pacific) and Implications for Upper Mantle Heterogeneity

AbstractThe Macquarie Ridge Complex (MRC) on the Australia‐Pacific plate boundary south of New Zealand is an extinct mid‐ocean ridge that has experienced a complex tectonic history and produced highly heterogeneous mid‐ocean ridge basalts (MORBs). When and how seafloor spreading ceased along the proto‐Macquarie mid‐ocean ridge remain elusive, and it is unclear how the mantle source of MORBs is affected by the gradual cessation of seafloor spreading at mid‐ocean ridges. To constrain the tectonic evolution of the MRC, the mantle source variations for MORBs at dying mid‐ocean ridges, and the mechanisms of mantle enrichment and asthenospheric heterogeneities, we report 11 pyroxene, plagioclase, basaltic glass, groundmass, and sericite 40Ar/39Ar and one zircon U‐Pb ages for the MRC MORBs. Our data reveal that basalts from the MRC seamounts were erupted between 25.9 and 1.6 Ma and Macquarie Island at ∼10 Ma. Combined age and plate reconstruction results reveal that the cessation of seafloor spreading at the MRC generally propagated from south to north along the ridge. Basalts produced by the then dying Macquarie mid‐ocean ridge at different times on different seamounts/island show a large variation in isotopic compositions and there is no clear correlation between ages and isotopic ratios. The heterogeneity of mantle source for MORBs from the proto‐Macquarie mid‐ocean ridge suggests that the upper asthenospheric mantle is heterogeneous, and such heterogeneity becomes most obvious at dying mid‐ocean ridges where the degrees of partial melting are low and a large range of melt compositions are produced.

Geochemical Variation of Miocene Basalts within Shikoku Basin: Magma Source Compositions and Geodynamic Implications

Shikoku Basin is unique as being located within a trench-ridge-trench triple junction. Here, we report mineral compositions, major, trace-element, and Sr-Nd-Pb isotopic compositions of bulk-rocks from Sites C0012 (>18.9 Ma) and 1173 (13–15 Ma) of the Shikoku Basin. Samples from Sites C0012 and 1173 are tholeiitic in composition and display relative depletion in light rare earth elements (REEs) and enrichment in heavy REEs, generally similar to normal mid-ocean ridge basalts (N-MORB). Specifically, Site C0012 samples display more pronounced positive anomalies in Rb, Ba, K, Pb and Sr, and negative anomalies in Th, U, Nb, and Ta, as well as negative Nb relative to La and Th. Site 1173 basalts have relatively uniform Sr-Nd-Pb isotopic compositions, close to the end member of depleted mantle, while Site C0012 samples show slightly enriched Sr-Nd-Pb isotopic signature, indicating a possible involvement of enriched mantle 1 (EM1) and EM2 sources, which could be attributed to the metasomatism of the fluids released from the dehydrated subduction slab, but with the little involvement of subducted slab-derived sedimentary component. Additionally, the Shikoku Basin record the formation of the back-arc basin was a mantle conversion process from an island arc to a typical MORB. The formation of the Shikoku Basin is different from that of the adjacent Japan Sea and Parece Vela Basin, mainly in terms of the metasomatized subduction-related components, the nature of mantle source, and partial melting processes.

Tracking a continental deep subduction and exhumation from granulitized kyanite eclogites in the South Altyn Tagh, northern Qinghai-Tibet Plateau, China

Kyanite eclogites can provide insights into global orogenic processes within subduction-collision zones. Here we present an integrated study of the petrography, mineral chemistry, zircon U-Pb geochronology, and whole-rock geochemistry of granulitized kyanite eclogites from South Altyn Tagh, located at the northern margin of the Qinghai-Tibet plateau, China. Our results suggest that the rocks experienced eclogite-facies metamorphism (27–37 kbar, 1030–1068 °C), followed by a high pressure-ultra-high temperature (HP-UHT) stage (17–20 kbar, 945–1033 °C), isothermal decompression, and a medium pressure-(ultra)-high temperature (MP-(U)HT) (10–12 kbar, 810–950 °C) granulite-facies overprint.Zircon U-Pb dating yielded a weighted mean U-Pb age of ca. 500 Ma for the metamorphic domains, which is interpreted as the timing of HP-UHT metamorphism. In contrast, the age of ca. 800 Ma recorded by the inherited cores of zircon grains is considered to represent the protolith formation age. High Al2O3 and CaO and relatively low MgO and FeO contents, LREE enrichment and nearly flat HREE patterns, and enriched Sr-Nd isotopic compositions suggest that the protoliths of the granulitized kyanite eclogites were likely derived from an enriched mantle source affected by crustal contamination. These results, together with previous research on these associations suggested that the protoliths of granulitized kyanite eclogites and associated garnet peridotite in the Bashiwake area are derived from mafic-ultramafic intrusions composed of plagioclase-rich gabbroic rocks and olivine-rich cumulates. These mafic-ultramafic intrusions were possibly generated by plume-induced melting related to the rifting of the Rodinia supercontinent during the Neoproterozoic, and were later subjected to early Paleozoic metamorphism. The geodynamic model proposed envisages continental subduction followed by relamination of felsic and mafic-ultramafic associations at the base of the lower crust, which were subsequently exhumed, forming diapir-like bodies. Our results, combined with those from previous studies, not only provide insights into the processes of continental collision to exhumation, but also provide significant constraints on the geotectonic processes leading to the formation of HP rocks within subduction-collision orogens.

Hybrid granite magmatism during orogenic collapse in the Eastern Desert of Egypt: Inferences from whole-rock geochemistry and zircon U–Pb–Hf isotopes

Granites can provide detailed insights into crust formation and reworking during the different stages of orogenic cycles. New field and petrographic observations are integrated with whole-rock geochemical and zircon U-Pb and Hf isotopic data of granitic and subvolcanic felsite intrusions associated with the Atalla Shear Zone (ASZ). Formation of the ASZ is attributed to regional oblique convergence, late in the tectono-magmatic evolution of the Central Eastern Desert of Egypt. The new data reveal three stages of post-collisional felsic magmatism, with a change in magma type from I- to A-type granites. The evolution commenced with formation of quartz-monzodiorite and granodiorite at 639 ± 2 Ma (stage 1), followed by a suite of granodiorite to syenogranite intrusions between 619 ± 2 and 612 ± 2 Ma (stage 2), and ceased with the Atalla felsites at 607 ± 2 Ma (stage 3). During stages 1 and 2, the ASZ was intruded by hybrid granites as a result of mixing of lower crust material with mantle-derived mafic sanukitoid-like melts, in addition to nearly pure crustal melts during stage 2. The Atalla felsites, with distinct A-type affinities, are interpreted to have resulted from fractional crystallization of hybrid magmas in shallow crustal levels prior to emplacement. The identical initial Hf isotope compositions of all ASZ-associated magmatic rocks imply that the crust and mantle reservoirs had similar Hf isotopic composition at ~640–610 Ma (εHf(t) = 8 ± 1). This implies that mantle enrichment was controlled largely by subducted Neoproterozoic oceanic crust, volcanic arc, and syn-collisional rocks. The new U-Pb ages combined with the geochemical characteristics of the ASZ granitic rocks are consistent with a prolonged period of decompression melting of hitherto enriched mantle and lower crust. Melting was likely triggered by rejuvenation of thoroughgoing transcurrent shearing and shear-driven asthenospheric mantle upwelling during the orogenic collapse stage.

Geoquímica dos diques máficos da região leste da Província Aurífera Alta Floresta: inferências sobre a fonte mantélica e implicações geodinâmicas

O enxame de diques máficos de Peixoto de Azevedo e Nova Guarita, região Leste da Província Aurífera Alta Floresta (PAAF), compõe-se de basaltos toleíticos evoluídos. Tais rochas provêm de fonte enriquecida por fluidos originados durante a subducção de crosta oceânica recoberta por significativa quantidade de sedimentos de composição semelhante à média global de sedimentos subductados (global subducting sediment composition — GLOSS). O enxame de diques intrude granitos e gnaisses paleoproterozoicos dos arcos magmáticos Cuiú-Cuiú e Juruena, e possui idade de 1419 Ma. No Cráton Amazônico, suas características geoquímicas relacionadas à sua origem mantélica são muito semelhantes às dos diques máficos de Carajás e da Suíte Máfica Vespor, ambos paleoproterozoicos. São semelhantes em idade quando comparados aos diques de Nova Lacerda, assim como de outras regiões do Supercontinente Columbia, como os diques Bas Drâa Inlier, a W do Cráton da África, a soleira Midsommerso, a NE da Groenlândia, e os diques Michael Gabbro, na Província Grenville, no Canadá. Essas intrusões ocorreram entre 1,42 e 1,33 Ga, durante as fases finais de fragmentação do Supercontinente Columbia.

PRECISE AGES FOR LODE GOLD MINERALIZATION IN THE XIAOQINLING GOLD FIELD, SOUTHERN MARGIN OF THE NORTH CHINA CRATON: NEW CONSTRAINTS FROM IN SITU U-Pb DATING OF HYDROTHERMAL MONAZITE AND RUTILE

Abstract The Xiaoqinling gold field, located along the southern margin of the North China craton, is the second largest gold producer in China, which comprises more than 1,200 auriferous quartz veins with a proven gold reserve of at least 800 tons. Previously, the absolute age of the gold metallogenesis in this area has not been well defined due to the lack of suitable dating minerals. This study presents new in situ laser ablation-inductively coupled plasma-mass spectrometry U-Pb ages of coexisting hydrothermal monazite and rutile for the Fancha gold deposit in this area, which yielded 206Pb/238U ages of 127.5 ± 0.7 Ma (n = 65, mean square of weighted deviates [MSWD] = 1.8) and 129.7 ± 4.3 Ma (n = 37, MSWD = 1.4), respectively. Both ages overlap within analytical uncertainty at the 2σ level of significance, suggesting that both gold-bearing veins were emplaced at ca. 128 Ma. Mineralogical observations indicate that the monazite and rutile precipitated simultaneously with gold from the hydrothermal fluid. Our new data, combined with recently published monazite age, define a more precise gold episode, demonstrating that the gold endowment of the Xiaoqinling area was formed during a relatively brief period at ca. 130 to 127 Ma. We suggest that auriferous fluids were generated as a result of interactions between the enriched mantle and the lower crust, which was driven by westward flat slab subduction of the Paleo-Pacific plate during the late Mesozoic. The peak of lithospheric thinning during the postsubduction may have led to the rapid release of gold from the fertilized mantle. Consequently, the large number of gold-bearing veins in the Xiaoqinling area may ultimately be related to the tectonic evolution and mantle fluid processes that occurred during Early Cretaceous lithospheric extension.

Orosirian I-type calc-alkaline granitoids from northern Brazil: Petrogenetic implications for evolution of the central Amazonian Craton

New petrographic, lithochemical, in situ UPb geochronological and Nd isotopes data allowed to propose a petrotectonic model for 1.90–1.88 Ga I-type calc-alkaline granitoids (Terra Preta Massif - TPM, Água Branca Suite) from the southernmost Uatumã-Anauá Domain (UAD), Ventuari-Tapajós Province (VTP), central Amazonian Craton, Brazil. The main rock facies of the TPM (1898–1885 Ma) vary from quartz monzodiorite to syenogranite, which display dominantly metaluminous to weakly peraluminous composition. A quartz diorite displaying particular petrographic features can be associated with the TPM and it does not seem to have evolved by fractional crystallization. Field and petrographic evidences, supported by trace element modeling, suggest that the quartz diorite has an origin linked to mixing between the TPM granodioritic (60 wt%) and related quartz gabbro (40 wt%) magmas. Lithochemical data show that a great petrographic variation of the TPG's main facies was not caused by fractional crystallization and point to different degrees of melting of the enriched mantle. Main facies also indicate weak positive ƐNd(t) anomalies that suggests upwelling asthenospheric mantle that provided heat to remelt the lower continental crust. The dataset of Nd isotopes and inherited zircon crystals also suggests reworking of Rhyacian granulites (French Guyana Domain-FGD, Maroni-Itacaiúnas Province) and Orosirian granulites (Anauá Complex, UAD, VTP) and assimilation of metasedimentary rocks (Cauarane Group, Central Guyana Domain, VTP), collisional granitoids (Serra Dourada Suite, UAD, VTP) and Orosirian orthogneisses (Rio Urubu Complex, CGD, VTP). Sources modeling of lithospheric mantle indicated that it is possible to generate tholeiitic mafic and I-type calc-alkaline melts, by melting of 2.15 Ga komatiitic metabasalts from FGD. A melt fraction of 28 wt% of 2.03 Ga granulites was also able to generate a liquid compatible with the main facies of the TPM. Offset of the slab breakoff in the post-collisional setting caused the asthenospheric mantle input and mixture of depleted melts (highly positive ƐNdt) with enriched melts (negative ƐNdt), resulting in ƐNd(t) values close to zero, which renders the lithospheric mantle as the main isotopic reservoir for the TPM.

Petrogenesis of ca. 113 Ma volcanic rocks in the central Lhasa subterrane, southern Tibet: Implications for the tectonic setting and continental crustal reworking

Voluminous Early Cretaceous volcanic rocks in the central Lhasa subterrane provide an ideal opportunity for understanding the mantle–crust interaction and tectonic‐magmatic evolution of the Lhasa Terrane. Here, we report zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic data for the Early Cretaceous volcanic rocks, including high‐silica rhyolites (117.2 ± 1.1 Ma) and contemporaneous andesites (114.1 ± 0.8 Ma) in the Sailipu area, central Lhasa subterrane. All these rocks show enriched light rare earth elements (LREE), and radioactive heat‐generating elements (e.g., Th, U, K, and Pb) but depleted high‐field‐strength elements (HFSEs, e.g., Nb, Ta, P, and Ti). The different bulk‐rock Sr–Nd–Pb and zircon Hf isotopic compositions of the rhyolites and andesites suggest that these rocks have distinct magma sources and petrogenetic history rather than an identical source involving assimilation‐fractional crystallization process. The Sailipu high‐silica rhyolites exhibit the characteristics of fractional crystallization and have varying zircon εHf(t) values (−12.2 to +5.2), negative εNd(t) values (−9.5 to −1.6), high and variable initial Sr isotopic compositions ([87Sr/86Sr]i = 0.7047–0.7116) and radiogenic Pb isotopic signatures (206Pb/204Pb = 18.784–18.802, 207Pb/204Pb = 15.722–15.737, and 208Pb/204Pb = 39.382–39.492). A combined process of magma mixing (involving crustal‐derived felsic melts and mantle‐derived mafic melts) and subsequent fractional crystallization were mainly responsible for the formation of these high‐silica rhyolites. The Sailipu andesites show more enriched Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7088–0.7154, εNd(t) = −9.9 to −6.7] relative to rhyolites, and less radiogenic Pb isotopic compositions (206Pb/204Pb = 18.632–18.669, 207Pb/204Pb = 15.686–15.689, 208Pb/204Pb = 39.140–39.201). They were likely derived from partial melting of an enriched mantle wedge previously metasomatized by melts derived from subducted sediments. We interprete these Early Cretaceous volcanic rocks as the product of slab break‐off during the southward subduction of the Bangong‐Nujiang Tethyan oceanic seafloor. The dominantly positive εHf(t) and ancient TDMC ages (0.85–1.90 Ga) of Sailipu high‐silica rhyolites further imply that, under the stress relaxation regime, the mantle‐derived melts locally reworked the ancient basement of the central Lhasa subterrane.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Anisian Alkaline Basalts in Gejiu District, Yunnan Province

The Gejiu Anisian alkaline basalts (GAAB), distributed in the southern part of the Emeishan large igneous province (ELIP), are crucial to understand the tectonomagmatic activity during the Triassic. Geochronological, geochemical, and Sr-Nd-Pb isotopic analyses were systematically applied to explore the origin, petrogenesis, and tectonic setting of the GAAB, and how they relate to the ELIP. Zircon U-Pb dating set the eruption date at 244 Ma. Most of the samples belonged to alkaline basalts and had high TiO2 (2.14–3.23 wt.%) and MgO (4.43–19.58 wt.%) contents. Large ion lithophile elements (LILEs) were enriched relative to high field strength elements (HFSEs). The rare earth elements (REEs) and trace element signatures in the normalized diagrams were similar to oceanic island basalts (OIB) and Emeishan high-Ti basalts. These samples had consistent Sr-Nd isotope compositions: the initial 87Sr/86Sr values ranged from 0.7044 to 0.7048 and εNd(t) = 3.25–4.92. The Pb isotopes were more complex, the (206Pb/204Pb)t, (207Pb/204Pb)t, (208Pb/204Pb)t ratios were 17.493–18.197, 15.530–15.722, and 37.713–38.853, respectively. Our results indicate that the GAAB originated from the deeper enriched mantle with 5% to 15% partial melting of garnet lherzolite and a segregation depth of 2 to 4 GPa (60–120 km). During the formation of the GAAB, clinopyroxene and Ti-Fe oxides were fractionally crystallized with insignificant crustal contamination. The GAAB were formed in a extensional regime that was related to the Gejiu-Napo rift event in the Triassic.

Petrology of the late Triassic mafic volcanic rocks from the Antalya Complex, southern Turkey: evidence for mantle source characteristics during the Neotethyan rifting

The Antalya Complex in southern Turkey comprises a number of autochthonous and allochthonous units that originated from the Southern Neotethys. Late Triassic volcanic rocks are widespread in the Antalya Complex and are important for the onset of the rifting stage of the southern Neotethys. The studied Late Triassic volcanic rocks within the Antalya Complex are exposed in the southern part of Saklıkent (Antalya) region. They are represented by pillow, massive, and columnar-jointed lava flows with volcaniclastic breccias and pelagic limestone intercalations. Spilitic basalts exhibit intersertal, microlithic porphyritic, and ophitic textures and are represented by plagioclase, pyroxene, and olivine. Secondary phases are characterized by serpentine, calcite, chlorite, epidote, zeolite, and quartz. Based on Zr/Ti vs. Nb/Y ratios, the volcanic rocks are represented by alkaline basalts (Nb/Y = 1.54–2.82). A chondrite normalized REE diagram for the volcanic rocks displays significant LREE enrichment with respect to HREE ([La/Yb]N = 15.14–19.77). Trace element geochemistry of the studied rocks suggests that these rocks are more akin to ocean island basalt (OIB) and were formed by small degrees (~2–4%) of partial melting of an enriched mantle source (spinel + garnet-bearing lherzolite). The volcanic rocks of the Saklıkent region exhibit similarities to the Late Triassic volcanics of the Koçali Complex in SE Anatolia and the Mamonia Complex (Cyprus) in terms of their geochemical features. All evidence suggests that the Late Triassic alkaline volcanics in Antalya, Mamonia (Cyprus), and the Koçali (Adıyaman) Complexes were formed in an extensional environment at the continent-ocean transition zone during the rifting of the southern Neotethyan Ocean.