Underplating Of Mantle-derived Mafic Magmas Research Articles

Middle Cambrian granites in the Dunhuang Block (NW China) mark the early subduction of the southernmost Paleo-Asian Ocean

The formation and evolution of the Central Asian Orogenic Belt (CAOB) were closely associated with the opening and closure of the so-called “Paleo-Asian Ocean” (PAO). The subduction of the southern PAO is expected to have resulted in widespread Paleozoic arc magmatism in the southernmost CAOB. However, the precise location and timing of such subduction remain uncertain. The Dunhuang Block (DHB) in the southernmost CAOB is thought to have recorded such subduction. In this contribution, we report the results of the zircon U–Pb ages, Hf isotopes, and whole-rock geochemical data for the Lianghu, Dapokouzi, and Xiaowan plutons in the northeastern DHB, NW China. Zircon U–Pb dating reveals that these plutons formed at ca. 510 Ma (the Middle Cambrian) and may represent the oldest arc-related magmatism in the DHB. Geochemically, the Dapokouzi pluton is characterized by high SiO2 (70.7–71.2 wt%), Na2O (5.11–5.26 wt%), Al2O3 (15.5–15.6 wt%), and Sr (730–733 ppm) contents, high Sr/Y (380–398) ratios, moderate Mg# (49–50) values, low MgO (0.61 wt%), K2O (2.00–2.01 wt%) and Y (1.84–1.93 ppm) contents, and weakly positive Eu (1.39–1.44) anomalies, indicating geochemical affinities of adakitic rocks. They have positive εHf(t) values (+11.2 to +13.9) close to the depleted mantle line and young two-stage model ages (TDM2 = 571–742 Ma). All the geochemical characteristics show that the Dapokouzi pluton was derived from the remelting of juvenile crust with addition of minor metasomatized mantle-derived materials and a residue phase dominated by garnet-bearing amphibolite. Similarly, the Lianghu and Xiaowan plutons have high SiO2 (70.2–74.4 wt%) and Al2O3 (12.8–14.3 wt%), low MgO (0.61–1.01 wt%) with Mg# values of 35–43 and K2O (0.43–1.55 wt%) contents. Compared to the Dapokouzi pluton, they exhibit low Sr (121–155 ppm) and Y (15.3–23.1 ppm) contents, low Sr/Y (5.25–9.54) ratios and Cr-Ni-Co concentrations, and pronouncedly positive εHf(t) values (+11.0 to +14.7), indicating that the magmas originated from the remelting of juvenile lower crust followed by significant fractional crystallization at relatively shallow crustal depths. Data from this and previous studies convincingly suggest the DHB was located in a thickened continental arc setting during the early Paleozoic, probably related to the early subduction of the PAO lithosphere. Crustal thickening, as indicated by the geochemical signature of the Dapokouzi pluton, was most likely associated with the underplating of mantle-derived mafic magmas during the slab subduction. This study therefore highlights that the subduction of the southern PAO might have initiated prior to 510 Ma, possibly dating back to the early Cambrian.

Petrogenesis and mineralization of the porphyry and skarn mineralization-related Husite intrusion in the Boluokenu metallogenic belt, Western Tianshan Orogenic Belt, NW China

Many porphyry and skarn Cu (Fe, Mo, Au, Zn) deposits have been discovered along the western Tianshan Orogenic Belt within the Central Asian Orogenic Belt. Generally, these Cu polymetallic deposits are considered to be linked to porphyries that formed in an arc setting, e.g., in a continental magmatic arc and island magmatic arc, but an agreement on their source region has not been reached. The Husite granitic complex and associated Kekesala skarn Fe-Cu deposit from the Boluokenu metallogenic belt provide significant insights into a comprehensive study of the ore-forming porphyries in Boluokenu. The ore-forming porphyries are mainly granodiorites and belong to calc-alkaline and high-K calc-alkaline series. LA-ICP-MS zircon U-Pb dating on the granodiorites yields ages of 368 Ma. The trace element compositions of the granodiorites are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE) and heavy rare earth elements (HREE), and slightly negative Eu anomalies and are similar to rocks that form in a continental arc setting. These rocks also show large variations in their εHf (t) (+4.1 to +9.7) and Mg# (38–69) values, alongside low Ba/Th (4–230) and high Th/Yb (3.77–7.41) and Th/La (0.21–0.71) ratios, suggesting that the granodiorite magma originated from the partial melting of mafic lower crust with a mantle wedge component and involved signals of subduction-zone sediments and fluids. The Husite mafic microgranular enclaves (MMEs) exhibit high Mg# (50–62), MgO (3.25–6.93 wt%) and low SiO2 (48.8–59.5%) contents, with positive εHf(t) from +3.3 to +7.4. The zircon LA-ICP-MS U-Pb ages of the MMEs (373 Ma) indicate nearly coeval formation between the MMEs and HMPs during the Late Devonian. These features with the petrological and mineralogical characteristics (such as needle-like apatite and disequilibrium feldspar xenocrysts) suggest that the Husite MMEs formed from the mixing of felsic magma from the partial melting of mafic lower crust and mafic magma that was derived from mantle wedge, rather than restite, xenoliths or fractional crystallization of magma. Thus, the skarn Fe-Cu mineralization were probably related to hybrid intermediate-acid magma that is characterized by high water and oxygen contents and high sulfur fugacity. It can be inferred that the response to ongoing slab roll-back during the Junggar plate’s subduction beneath the Yili Block, including the underplating of mantle-derived mafic magmas from the metasomatism of subduction-zone melts, led to the partial melting of the mafic lower crust to form felsic magma. Then, the continuous reaction between the mafic and felsic magmas further increased the Fe-Cu concentrations and was closely related to the subsequent Fe-Cu mineralization.

Ages and petrogenesis of Jurassic and Cretaceous intrusive rocks in the Matsu Islands: Implications for lower crust modification beneath southeastern China

Major and trace element, whole-rock Sr-, Nd- and Hf-isotope, zircon U-Pb age and Hf-O isotope data are reported for the intrusive rocks from the Matsu Islands in the coastal area of southeastern (SE) China, in order to study the ages, sources and petrogenesis of these rocks and evolution of the lower crust. The rocks include gneissic granite, massive granite, brecciated granite and diabase. Secondary ion mass spectrometer (SIMS) zircon U-Pb dating reveals that the rocks in the Matsu Islands were emplaced at ∼160 Ma, ∼130 Ma and ∼94 Ma. The Jurassic granites (∼160 Ma) have high SiO2 (74.1–74.5 wt%) and K2O+Na2O (8.32–8.33 wt%) contents and high Rb/Sr ratios of 0.6–1.2 and (La/Yb)CN ratios of 12.6–19.4. Their relatively high initial 87Sr/86Sr ratios (0.7074–0.7101), variable and negative εNd(t) values (−9.2 to −5.4), and variable zircon εHf(t) (−17.0 to +5.2) and δ18O (4.7–8.1‰) values indicate they were mainly derived from an ancient lower crustal source, but with involvement of high εHf(t) and low δ18O materials. The Early Cretaceous diabase (∼130 Ma) has SiO2 content of 56.5 wt%, relatively high MgO concentration, low initial 87Sr/86Sr ratio and negative εNd(t) value, similar to geochemical features of other Cretaceous mafic rocks in the coastal area of SE China. Zircons from the diabase have high εHf(t) values (−5.5 to +0.2) and relatively low δ18O values of 4.2–5.0‰. These characteristics indicate that the parental magma of the diabase was generated by partial melting of enriched lithospheric mantle, which have been metasomatised by altered oceanic crust-derived low-δ18O fluids. For the Cretaceous granitoids (∼130 Ma and 94 Ma), they have relatively low SiO2 (68.0–71.3 wt%) and K2O+Na2O (5.30–7.55 wt%) contents and low Rb/Sr ratios and (La/Yb)CN ratios of 5.8–7.1. They have low initial 87Sr/86Sr ratios (0.7071–0.7082), homogeneous εNd(t) (−4.3 to −4.5) and relatively high zircon εHf(t) values (−3.7 to +1.2) and low δ18O values (4.6–5.9‰). Their isotopic compositions are similar to those of the diabases in this study as well as other Cretaceous mafic rocks in the coastal area of SE China, suggesting that the sources of the Cretaceous granitoids might be the newly formed lower crust related to the underplated mafic rocks. Whole-rock geochemical, Sr-Nd and zircon Hf-O isotopic compositions indicate that the Jurassic granitoids are most likely generated by partial melting of relatively ancient basement rocks, whereas the Cretaceous granitoids were generated by partial melting of relatively young lower crustal rocks with addition of mantle-derived magma. This distinction implies that the pre-existing ancient lower crust beneath the coastal area of SE China has been modified by large-scale mafic magma underplating. Therefore, underplating of mantle-derived mafic magma would result in modification of the pre-existing ancient lower crust and formation of the relatively juvenile lower crust.

Early Cretaceous Na-rich granitoids and their enclaves in the Tengchong Block, SW China: Magmatism in relation to subduction of the Bangong–Nujiang Tethys ocean

The Na-rich intermediate-to-felsic granitic rocks provide insights into the generation of magmas in subduction zones. This paper presents zircon LA-ICP-MS U–Pb ages as well as whole-rock geochemical, mineral chemical, and in situ zircon Hf isotopic data on Na-rich granitic rocks from the Tengchong Block, SW China. The granodiorites and associated mafic magmatic enclaves (MMEs) from the Menglian batholith yield zircon U–Pb ages of 116.1±0.8 to 117.8±0.6Ma and 117.7±0.7Ma, respectively. Both host granodiorites and enclaves show calc-alkaline and sodium-rich nature, enrichment in large-ion lithophile elements (LILEs), and variable depletion in zircon Hf isotopic compositions. Euhedral amphiboles in both granodiorites and associated enclaves are magnesian-hornblende with high Mg and Ca and contain euhedral plagioclase inclusions of labradorite to andesine (An36–57) composition. The granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources. The quartz monzodiorite–granodiorites and associated MMEs from the Xiaotang-Mangdong batholith yield zircon U–Pb ages of 120.3±1.3 to 122.6±0.8Ma and 120.7±1.5Ma. These rocks are also sodium-rich and show calc-alkaline trend with negative zircon Hf isotopic compositions (−5.55 to +0.58). The MMEs in the host intrusions are monzogabbro with variable and depleted zircon Hf isotopic compositions. The amphiboles in the both host intrusions and the enclaves show Al-rich ferro-tschermakite composition. We infer that the quartz monzodiorite–granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle-derived mafic magmas. The formation of the different types of Na-rich granitic rocks is correlated to the subduction of Bangong–Nujiang Tethyan ocean. A comparison with magmatism in the northern magmatic belt suggests that mantle-derived magmas played an important role in the genesis of Early Cretaceous intrusions from Tengchong to Lhasa Blocks, although crustal melting is the dominant contributor.

Zircon U–Pb dating, geochemical and Sr–Nd–Hf isotopic characteristics of the Jintonghu monzonitic rocks in western Fujian Province, South China: Implication for Cretaceous crust–mantle interactions and lithospheric extension

Comprehensive petrological, in situ zircon U–Pb dating, Ti-in-zircon temperature and Hf isotopic compositions, whole rock geochemical and Sr–Nd isotopic data are reported for the Jintonghu monzonitic intrusions in the western Fujian Province (Interior Cathaysia Block), South China. The Jintonghu monzonitic intrusions were intruded at 95–96Ma. Their Sr–Nd–Hf isotopic compositions are similar to the coeval and nearby enriched lithospheric mantle-derived mafic and syenitic rocks, indicating that the Jintonghu monzonitic rocks were likely derived from partial melting of the enriched mantle sources. Their high Nb/Ta ratios (average 21.6) suggest that the metasomatically enriched mantle components were involved, which was attributed to the modification of slab-derived fluid and melt by the subduction of the paleo-Pacific Plate. The presence of mafic xenoliths, together with geochemical and isotopic features indicates a mafic-felsic magma mixing. Furthermore, the Jintonghu intrusions may have experienced orthopyroxene-, biotite- and plagioclase-dominated crystallization. Crust–mantle interaction can be identified as two stages, including that the Early Cretaceous mantle metasomatism and lithospheric extension resulted from the paleo-Pacific slab subduction coupled with slab rollback, and the Late Cretaceous crustal activation and enhanced extension induced by dip-angle subduction and the underplating of mantle-derived mafic magma.

Anticlockwise P-T evolution at ∼280 Ma recorded from ultrahigh-temperature metapelitic granulite in the Chinese Altai orogenic belt, a possible link with the Tarim mantle plume?

An ultrahigh-temperature (UHT) metapelitic granulite assemblage consisting of garnet(g)–spinel(sp)–orthopyroxene(opx)–sillimanite(sil)–cordierite(cd)–ilmenite(ilm)–biotite(bi)–plagioclase(pl)–quartz(q) occurs within migmatitic paragneiss near Kalasu in the Chinese Altai, NW China. Textural relations, mineral compositions and P-T estimates, indicate three stages of mineral assemblages: (1) pre-peak prograde stage (M1) consisting of a sp–sil-bearing or sp–opx-bearing inclusion assemblage, with low-Al2O3 contents (4–5wt.%) in orthopyroxene and P-T conditions of ∼7kbar and ∼890°C, (2) peak UHT stage (M2) comprising a g–opx–cd-bearing coarse-grained assemblage, with high-Al2O3 contents (8–9wt.%) in orthopyroxene and peak conditions of ∼8kbar and ∼970°C, and (3) post-peak HT stage (M3) containing an oriented opx–bi–sil-bearing assemblage in matrix, with moderate amounts of Al2O3 (6–7wt.%) in orthopyroxene and P-T conditions of 8–9kbar and ∼870°C. The three discrete stages define an anticlockwise P-T path involving initial prograde heating and post-peak near isobaric cooling. Such a near isobaric cooling anticlockwise P-T path suggests that UHT metamorphism likely occurred in an overall extensional tectonic setting with associated underplating of mantle-derived mafic magma. A SHRIMP zircon U–Pb age of 278±2Ma obtained from the metapelitic granulite indicates UHT metamorphism in the Altai orogen occurred during the Permian, coeval with spacially associated mantle-derived mafic intrusions (∼280Ma) and the Tarim mantle plume (∼275Ma). Thus, the Permian UHT metamorphism of the Chinese Altai is likely associated with underplating and heating of mantle-derived mafic magma as a result of the Tarim mantle plume.

Late Cretaceous crustal growth in the Gangdese area, southern Tibet: Petrological and Sr–Nd–Hf–O isotopic evidence from Zhengga diorite–gabbro

Recent studies of Gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern Tibet during the Jurassic–Early Eocene. However, the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes. Here, we report on the Zhengga diorite–gabbro suite from the Gangdese area. They consist of plagioclase (labradorite and anorthite), amphibole, clinopyroxene, biotite and minor magnetite, epidote, zircon and apatite. LA-ICP-MS zircon U–Pb dating for two samples gives a ca. 94Ma age for the Zhengga intrusive rocks, i.e., the Late Cretaceous. Apart from one high-SiO2 (52.2wt.%) diorite sample with slightly high K2O (1.72wt.%) and initial 87Sr/86Sr (0.7068) and low εNd(t) (−5.6) values, the gabbro samples are geochemically characterized by low SiO2 (39.8–50.1wt.%) and K2O (0.3–1.1wt.%), strongly negative Nb–Ta and positive Sr anomalies, and uniform initial 87Sr/86Sr (0.7043–0.7048). The gabbros can be divided into two groups: Group I gabbros with relatively low total rare earth element (REE), Rb and SiO2 contents and positive Eu anomalies, and Group II gabbros with slightly higher total REE, Rb and SiO2 contents and negative Eu anomalies. The Group I gabbros have εNd(t) (+1.7 to +4.1), and zircon εHf(t) (+6.5 to +11.1) and δ18O (5.89 to 7.24‰) values, which are slightly different to those of the Group II gabbros (−0.2 to +2.0, +2.9 to +6.5 and 6.24 to 7.05‰). Trace element compositions of amphibole and clinopyroxene grains suggest that the Zhengga mafic magmas contained a significant fluid-transported component, probably released from subducted oceanic lithosphere. We suggest that the parental magmas of the Zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts/fluids. The Group I gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas, with insignificant crustal contamination, whereas the Group II gabbros were probably produced by assimilation and fractional crystallization (AFC) processes from mafic magmas that were geochemically similar to the Group I gabbros. Pre-collisional underplating of mantle-derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late Late Cretaceous–Cenozoic granitoids of the Gangdese batholiths. This study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust.

Association of Neoproterozoic A- and I-type granites in South China: Implications for generation of A-type granites in a subduction-related environment

Neoproterozoic magmatism in the Yangtze Block of South China produced voluminous S- and I-type granites, and sparse A-type granites. The Daxiangling A-type granitic pluton is spatially associated with the Shimian I-type pluton at the western margin of the Yangtze Block. Both plutons have similar SHRIMP zircon U–Pb ages of ∼ 800 Ma and are slightly younger than the tonalite–trondhjemite–granodiorite (TTG) gneisses in the area. The Shimian pluton is composed of granite and monzogranite with high SiO 2 (69.3–76.6 wt.%), Na 2O (2.79–3.80 wt.%) and K 2O (3.94–5.87 wt.%), and low Fe 2O 3 (0.96–3.06 wt.%) and MgO (0.12–0.50 wt.%). The Daxiangling pluton consists of alkali–feldspar granites with higher SiO 2 (76.3–79.3 wt.%) and lower Al 2O 3 (10.6–11.9 wt.%) and CaO (0.21–0.55 wt.%) than the Shimian granites. Both plutons are slightly peraluminous (A/CNK = 1.00–1.12) and belong to the high-K, calc-alkaline series. The Daxiangling alkali-feldspar granites have much higher Zr, Hf, Ga and HREE, and lower Sr than the Shimian granites. On the chondrite-normalized REE diagram, both plutons are enriched in LREE, but the rocks of the Daxiangling pluton show relatively flat patterns because of their higher HREE contents. Their primitive-mantle normalized spidergrams display negative Eu, Nb, Ta, Sr, P and Ti anomalies. Both plutons have nearly identical whole-rock Nd and zircon Hf isotopic compositions ( ε Nd( t) = + 1 and ε Hf( t) = + 5 to + 9), similar to the TTG gneisses. We suggest that the I-type granites of the Shimian pluton were produced by dehydration melting of the TTG rocks as a result of underplating of mantle-derived mafic magmas. The rocks above the I-type magma source may have been converted to charnockites by heating and dehydration. With increasing temperatures, the charnockites underwent partial melting at temperatures > 900 °C to produce A-type magmas. Both the A- and I-type granites formed at an extensional active continental margin, perhaps in a back-arc environment. A-type granites with mantle-like isotopic signatures may be reworked or recycled from juvenile crustal rocks. The association of A- and I-type granites suggests that aluminous A-type granite is most probably derived from a charnockite source heated by large-scale magmatic underplating, rather than a tonalite source.

Geochronologic constraints on magmatic intrusions and mineralization of the Zhunuo porphyry copper deposit in Gangdese, Tibet

In situ zircon U-Pb ages for the recently discovered Zhunuo porphyry copper deposit in the western part of the Gangdese metallogenic belt in Tibet were determined by sensitive high-resolution ion microprobe (SHRIMP). The ages can be divided into two separate groups, reflecting more than four major tectono-magmatic events in the area. The 62.5±2.5 Ma age of inherited zircons may be related to the volcanic eruption of the Linzizong Group formed shortly after the India-Asia continental collision. The 50.1±3.6 Ma age most likely corresponds to the time of underplating of mantle-derived mafic magma in Gangdese. The 15.6±0.6 Ma age obtained from magmatic zircons is interpreted as the age of crystallization of the Zhunuo ore-forming porphyry. Finally, a molybdenite Re-Os isochron age of 13.72±0.62 Ma is consistent with another zircon U-Pb age of 13.3±0.2 Ma, representing the time of copper mineralization. These ages, in combination with available literature data, indicate that magmatic crystallization and copper mineralization in the Gangdese metallogenic belt became gradually younger westward, and further suggest that the Zhunuo porphyry copper deposit was formed in the same tectonic stage as other porphyry copper deposits in the eastern and central Gangdese belt. This conclusion provides critical information for future exploration of porphyry copper deposits in western Gangdese.

Geochemistry and geochronology of high-grade rocks from the Grove Mountains, East Antarctica: Evidence for an Early Neoproterozoic basement metamorphosed during a single Late Neoproterozoic/Cambrian tectonic cycle

The Grove Mountains of East Antarctica are an inland continuation of the Prydz Belt. The high-grade metamorphic complex in this area is composed of felsic orthogneisses and mafic granulites, with minor paragneisses and calc-silicate rocks. U–Pb zircon analyses using SHRIMP and LA-ICP-MS techniques reveal that the protoliths of mafic granulites and felsic orthogneisses were emplaced during a short time interval of ca. 920–910 Ma. Mafic granulites can be divided into low- and high-Ti groups. They have initial ɛ Nd values [ ɛ Nd( T)] ranging from +0.8 to −1.9. TiO 2 is positively correlated with FeO t/MgO and La/Nb ratios, whereas it shows a negative correlation with ɛ Nd( T) values, indicating that the petrogenesis of their protoliths involved partial melting of a weakly enriched subcontinental lithospheric mantle and fractional crystallization of the magma accompanied by minor crustal contamination. Felsic orthogneisses have an affinity of A 2-type granites, characterized by enrichment in REE, Y, Zr, Th and Ga and high Ga/Al and Y/Nb ratios. Most of them have ɛ Nd( T) values of −0.7 to −3.5 and Nd depleted mantle model ages ( T DM) of 1.76–1.65 Ga, and a few have low ɛ Nd( T) values of −10.4 to −10.6 and old T DM of 2.46–2.27 Ga, reflecting a heterogeneity in their source region. Their protoliths were probably produced by high-temperature partial melting of tonalitic–granodioritic rocks triggered by the underplating of mantle-derived mafic magma during post-orogenic extension. U–Pb analyses also reveal a metamorphic age of ca. 2050 Ma from detrital zircons in a paragneiss, suggesting that the sedimentary materials of the paragneiss may have come from an as yet undiscovered Early Paleoproterozoic orogen of unknown provenance. Voluminous mafic–felsic intrusives and a small amount of sedimentary rocks constitute an Early Neoproterozoic basement of the Grove Mountains. Subsequently, this basement experienced only a single Late Neoproterozoic/Cambrian metamorphic cycle at ca. 550–535 Ma. The available data indicate that the Prydz Belt is a collage of multiple basement terranes and each of them has a distinct tectonic evolution. This supports the suggestion that the Prydz Belt may represent a Late Neoproterozoic/Cambrian collisional orogen that resulted in the final phase of the Gondwana assembly.