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Early Cretaceous granitoids are widespread on the Tibetan Plateau and record the tectonic evolution of several Tethyan oceans. We describe Early Cretaceous high-Mg (Mg# > 50) and low-Mg (Mg#

Cretaceous granites are widely exposed in the Tengchong Terrane that is the southern extension of the Tibetan Plateau, resulting from the evolution of the Tethys. They, therefore, play a critical role in deciphering the Tethys evolution and the growth of the Tibetan Plateau. In this paper, we present new zircon U Pb dating and Hf isotopic results, along with whole-rock elemental and Sr Nd isotopic data of the granitoids from the Lushui-Pianma (LP) batholith and its mafic enclaves and the strongly deformed granites from the Gaoligong shear zone in the northern Tengchong Terrane. The dating results show that both the LP granitic batholith and the strongly deformed granites in the Gaoligong shear zone formed in the Early Cretaceous of 122–110 Ma. They are composed of granodiorite, quartz monzodiorite and monzogranite with minor gabbroic enclaves. Mineralogically and geochemically, the granitoids in the LP batholith, particularly for the less evolved ones, display a metaluminous nature and an affinity to I-type granite. All the granitoids are characterized by similar rare and trace patterns and whole-rock Nd and zircon Hf isotopic compositions, indicating a common origin. The sodium-rich feature for the less evolved dioritic rocks and the predominantly negative zircon eHf(t) and whole-rock eNd(t) values for all the granitoids demonstrate that they were derived mainly from the partial melting of the Mesoproterozoic metabasic rocks (likely the basement rocks of the Gaoligong Group) in the lower crust of the Tengchong Terrane. In the case of the mafic enclaves, they share similar mineralogical assemblages and Nd Hf isotopic compositions to its host granodiorite, indicating a cognate origin with the latter. In combination with previous data for the granitoids in the Tengchong Terrane, our study further illustrates that the Early Cretaceous granitic rocks are dominated by metaluminous I-type granites with minor highly fractionated peraluminous I-type granites, rather than the dominant S-type granites as assumed before. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar, together with regional Early Cretaceous magmatic and sedimentation patterns, indicate that these Early Cretaceous magmatic rocks were the products of the evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab breakoff.

Cenozoic basalts in SE China may be derived from a mixture of depleted MORB mantle (DMM) and enriched mantle 2 (EM2) sources, but whether these basalts share a common mantle source or magmatic history remains unknown. To investigate these unresolved issues, this study sampled basalts from Niutoushan and Mingxi (Fujian province), Xilong (Zhejiang province), and Penghu (Taiwan) for geochemical analysis. The basalt samples show OIB-like trace element patterns and have low PGE contents, with 0.02–0.7 ppb Ir and Pd, 0.05–1.4 ppb Ru, 0.01–0.2 ppb Rh, and 0.06–1.1 ppb Pt. All samples have high Cu/Pd ratios ranging from ~ 69,000 to 3,500,000, and low Cu/Zr ratios ranging from 0.1 to 0.8, suggesting sulfur-saturated fractionation. Model calculations indicate that the basalts are depleted in PGE due to the retention of 0.001% to 0.1% sulfide in the mantle and the removal of up to 0.0022% sulfide during magma ascent. The crystallization of olivine and spinel, and partial melting are insufficient to account for the observed PGE variation in these basalts. Thus, the distinct PGE patterns in basalts with different ages may reflect the heterogeneity of the mantle source beneath SE China. The source heterogeneity may be due to compositional heterogeneity, particularly variations in oxygen fugacity and PGE mineral phases, or due to variable fluid/melt metasomatic agents in the sub-continental lithospheric mantle. This heterogeneity is possibly related to the westward subduction of the Paleo-Pacific Plate.

High-Mg diorites were discovered in the southern part of the ca. 830 Ma Dongma Pluton, northern Guangxi Province of southern China. The diorites (SiO 2 = 59–65 wt%) are characterized by high MgO (6.7–8.9 wt%) contents and Mg-number [Mg# = 100 × Mg/(Mg + Fe)] (69–73), in contrary to the associated medium-Mg (MgO = 3.4–3.8 wt%, Mg# = 59–63) granodiorites in the Dongma main body and the low-Mg (MgO = 1.4–1.9 wt%, Mg# = 46–51) granodiorites in the Bendong Pluton to the north. Moreover, the high-Mg diorites show surprisingly high Cr (595–640 ppm) and Ni (171–194 ppm) concentrations, which are beyond the ranges of most coeval mafic rocks in the study area. Correspondingly, chromite crystals were separated from the high-Mg diorites and some of the medium-Mg granodiorites, and they show high Cr# [100 × Cr/(Cr + Al)] (average of 75), but low Mg# (0.34–2.51) and low Fe 3 + . The decoupling of Cr# and Mg# and the existence of quartz + apatite mineral inclusion in chromites suggest Mg-Fe exchange that may be facilitated by the disequilibrium resulted from magma mixing. The high-Mg diorites show low La/Yb (6.8–8.5) and Sr/Y (2.1–3.1) ratios, significant negative anomalies of Nb and Ti and positive anomaly of Pb, resembling the Setouchi high-Mg andesites, despite of their relatively low Sr (71–100 ppm). All of the studied diorites and granodiorites show enriched Nd isotope compositions, with e Nd (t) values (− 3.2 to − 5.9) a bit higher than some of the associated mafic rocks. Some of the high-Mg diorites show whole-rock e Hf (t) (− 6.0 to − 6.2) coupled with Nd isotopes, similar to the associated mafic-ultramafic rocks in northern Guangxi, suggesting the metasomatism by melts of subducting sediments in the mantle source. Whereas, others show decoupled Nd-Hf isotopes that are similar to the medium- and low-Mg granodiorites [e Hf (t) = − 1.8 to + 0.05], probably indicating the late magma mixing with granitic magmas at a crustal level for the dioritic magmas. We propose a two-stage model for the petrogenesis of the high-Mg diorites: 1) the mantle source was firstly metasomatized by melts from partial melting of subducting terrigenous sediments to form the enriched Nd-Hf isotopic characteristics; and then 2) the mantle-derived high-Mg mafic melts mixed with the crust-derived low-Mg granitic melts to form the high-Mg diorites and medium-Mg granodiorites. The occurrence of high-Mg diorites implies the existence of Neoproterozoic subduction-related metasomatism in the western part of the Jiangnan orogen.

The Baima mafic–ultramafic layered intrusion of the 260-Ma Emeishan Large Igneous Province (ELIP) hosts the second largest Fe–Ti–(V) oxide deposit in the Panxi region, SW China. It is a ~ 1600-m-thick layered body intruded by slightly younger syenitic and granitic plutons. The intrusion includes the Lower and Upper Zones. Troctolite and olivine pyroxenite of the Lower Zone contains conformable oxide ore layers, whereas the Upper Zone consists of olivine gabbro and gabbro with abundant apatite in the higher level. The crystallization order of the silicates in the Baima intrusion is olivine → plagioclase → clinopyroxene. Fe–Ti oxides (titanomagnetite and ilmenite) crystallized after olivine, and possibly plagioclase. The oxide ores in the Lower Zone show slightly LREE enriched patterns with (La/Yb)N values between 2.0 and 6.4, and positive Eu anomalies (Eu/Eu⁎) of 1.0 to 2.7. In contrast, olivine gabbros in the Lower Zone display stronger LREE enrichments (La/YbN = 7.7–14.0) and positive Eu anomalies (Eu/Eu⁎ = 2.8–3.3). Gabbros in the Upper Zone have REE profiles characterized by intermediate LREE enrichments with (La/Yb)N values of 3.2 to 11.2 and positive Eu anomalies of 2.1 to 3.0. Primitive mantle-normalized trace element patterns are characterized by negative La–Ce, Th, Sm and positive Nb–Ta, Ba and Ti anomalies in oxide ores and negative Th–U, Zr–Hf and positive Ba, Sr and Ti anomalies in olivine gabbro and gabbro. Fo of olivine and An of plagioclase remain roughly constant from 0 to ~ 90 m in the Lower Zone, indicating that the magma chamber was continuously filled by compositionally similar magmas during the initial stage. Three magma replenishments occurred afterwards in the upper part of the Lower Zone and the Upper Zone based on compositional reversals of plagioclase, olivine and Sr isotope. Mass balance calculations show that the Baima parental magma can produce all oxide ores under closed system conditions. A wide range of An values of plagioclase within thin sections and disequilibrium Sr isotopic compositions of plagioclase both along the stratigraphic profile (87Sr/86Sri = 0.70312–0.70510) and within thin sections indicate convection and co-accumulation of cumulus plagioclase that had crystallized from different magmas. We propose that the ~ 1600m-thick Baima intrusion formed in an open and isotopically heterogeneous magma chamber that was periodically recharged by compositionally similar or more primitive magmas. In each replenishment cycle, the magma underwent progressive crustal contamination and fractional crystallization. Convection and sorting of crystals based on density differences result in igneous layering characterized by intervals of oxide ores, troctolite and olivine gabbro.

Triassic Paleo-Tethyan subduction-related volcanic rocks are extensively distributed in the Xiangcheng and Changtai regions, northern Yidun Terrane, eastern Tibet. The ~ 228 Ma volcanic rocks in the Xiangcheng region have intermediate-felsic composition (60.1 to 64.7 wt.% SiO 2 ) with relatively low Y (14–18 ppm) and high Sr (630–1830 ppm) yielding high Sr/Y ratios (43–94). They have LREE-enriched and HREE-depleted REE patterns (La/Yb = 19–26), typical of adakites. Sr–Nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern Yidun Terrane. The ~ 231–230 Ma volcanic rocks in the Changtai region are composed of basalts, andesites, dacites and rhyolites with SiO 2 ranging from 47.7 to 81.0 wt.%. Basalts in the Changtai region have initial 87 Sr/ 86 Sr ratios from 0.7052 to 0.7058 and e Nd (t) from + 1.1 to + 1.5. They display OIB-like geochemical signatures, and were likely derived from low degrees of partial melting of an OIB-like mantle source with subordinate input of subduction components indicated by increasing Th content. Andesites and dacites show elemental and isotopic compositions that evolved from the basalts, indicative of derivation via crustal contamination and fractional crystallization (AFC) of basaltic magmas. Rhyolites have lower initial 87 Sr/ 86 Sr ratios and higher e Nd (t) values than andesites/dacites, inconsistent with an AFC process and, are suggested to have been derived from anatexis of crustal materials. Emplacement of volcanic rocks in the Changtai and Xiangcheng regions could be attributed to the subduction of the Ganzi–Litang Ocean, a branch of the Paleo-Tethys Ocean. Volcanic rocks in the Changtai and Xiangchang regions have zircon U–Pb ages about 4–6 myr older than that of arc granites in the eastern Yidun Terrane. The spatio-temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern Yidun Terrane in the Middle Triassic and extended to the northern Yidun Terrane at the beginning of the Late Triassic.

The Kalatongke complex, located in south Chinese Altay of the Central Asian Orogenic Belt (CAOB), consists of the ~ 308 Ma dioritic suite intruded by the 287 Ma noritic suite. The noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization. The dioritic suite is composed of ferrodiorite, diorite and quartz diorite. Ferrodiorites have 43.6 to 51.8 wt.% SiO 2 , 8.2 to 12.1 wt.% Fe 2 O 3 T , 2.87 to 4.86 wt.% MgO, with Mg# ranging from 42 to 53. They have highly variable Al 2 O 3 (17.4–20.7 wt.%), Na 2 O (2.78–4.50%), K 2 O (0.58–3.70%), Sr (280–1300 ppm) and Ba (219–3100 ppm). Diorites and quartz diorites have 15.2–19.6 wt.% Al 2 O 3 , 239–890 ppm Sr and170–1100 ppm Ba, 11.2–25.5 ppm Y and1.2–2.2 ppm Yb. All rocks have LREE enriched REE patterns and do not display Eu anomalies. They have primitive mantle-normalized trace element patterns enriched in large ion lithosphile elements (LILE) (e.g. Ba and Sr) and depleted in Ti, Nb and Ta. Both ferrodiorites and diorites have relatively constant initial 87 Sr/ 86 Sr ratios of 0.7037 to 0.7040 but highly variable e Nd(t) values of + 4.5 to + 11.6, whereas quartz diorites have higher initial 87 Sr/ 86 Sr ratios (0.7056) and lower e Nd(t) values (+ 0.5). Plagioclase from ferrodiorites, diorites and quartz diorites has similar initial 87 Sr/ 86 Sr ratios of 0.7015 to 0.7055. The dioritic suite formed from a mixed magma of an evolved mantle-derived basaltic magma by partial melting of subduction-modified mantle source in response to slab breakoff and a crustal-derived granitic magma at ~ 308 Ma. Before mixing with granitic magmas, basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite. The mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene, amphibole and plagioclase. The noritic suite formed from a mantle-derived high-Mg magma in a post-collisional environment. Both the dioritic and noritic suites in Kalatongke suggest the switch from subduction to extension and the final collision between the Junggar Block and Chinese Altay at ~ 308 Ma.

Permian basalts are widely distributed in the Tarim Basin and surrounding areas of NW China. The magmatism is represented by basaltic flows in Keping and mafic and ultramafic dykes in Silurian–Devonian strata in Bachu, southwestern Tarim Basin. The basalts in Keping have SiO2 (44.1%–55.5 wt.%) and total alkalis (Na2O + K2O = 3.20–7.79 wt.%) similar to the mafic dykes in Bachu, but with much higher TiO2 (3.53–4.33 wt.%). An ultramafic dyke has relatively low SiO2 (44.6–43.2 wt.%) and high MgO (19.0–20.2 wt.%), reflecting the abundance of cumulate olivine. All the rocks, including both lavas and dykes, have parallel, mantle-normalized trace element patterns enriched in Rb, Ba, Th, Nb, Ta, Zr, Hf, and light rare earth elements (LREE). The basalts have higher initial 87Sr/86Sr ratios (0.7064 to 0.7080) and lower eNd(t) values (− 2.66 to − 9.27) than the dykes (initial 87Sr/86Sr ratios range from 0.7048 to 0.7052 and eNd(t) values from + 1.64 to + 5.16). Both the basalts and dykes show a narrow range of 206Pb/204Pb (17.87–18.77), 207Pb/204Pb (15.52–15.58) and 208Pb/204Pb (38.38–39.04) ratios. The dykes do not show significant crustal contamination and were derived from an OIB-like, asthenospheric mantle source. In contrast, the basaltic flows show variable degrees (up to 10%) of crustal contamination and were derived from an OIB-like, but isotopically more enriched, asthenospheric mantle source. Olivine from the ultramafic dyke has Fo values up to 85, corresponding to a melt temperature of 1300 °C and a melt Mg# of 63. The Tarim Basin magmatism reflects partial melting of heterogeneous mantle sources related to a major mantle plume. Spatially and temporally associated mafic–ultramafic and syenitic intrusions and volcanic rocks form the ~ 275 Ma Tarim large igneous province.