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Zircon U Pb analyses of granitic gneisses from the gneiss domes (Xiaru, Mabja, Lhagoi Kangri, and Kangmar)within the Tethyan Himalaya and from the High Himalaya (Gyirong, Yadong, and Namche Barwa)yield that their protolith ages range from 509 Ma to 429 Ma. Similar to the Himalayan Cenozoic leucogranites, the Paleozoic granites are characterized by (1) high SiO2 (69.9–77.0 wt%), Al2O3 (12.4–16.1 wt%), K2O + Na2O (5.8–10.3 wt%) with K2O/Na2O > 1.0 and A/CNK > 1.0; (2) large variations in Rb (166–851 ppm), Sr (20–172 ppm), Ba (27–1474 ppm) and Rb/Sr ratios (1.0–38.3); (3) low degrees of LREE over HREE fractionation with (La/Yb)N

Gabbroic rocks in continental arcs constrain the composition of sub-arc mantle and the roles played by mafic magmas in the generation and differentiation of continental crust. Here, we report whole-rock elemental and isotope (Sr and Nd) compositions of ~50 Ma and ~30 Ma gabbroic rocks from the central and eastern Gangdese (CG and EG) batholith, southern Tibet. A key observation is the along-arc isotopic and geochemical variations of ~50 Ma gabbros and dykes: EG rock samples show enriched Sr Nd isotope compositions, with initial 87Sr/86Sr = 0.7053–0.7077 and eNd(t) = −4.3 to +1.1 (average −3.0), and have higher abundance of heavy rare-earth elements (HREEs), high field strength elements, and melt-mobile elements compared with CG samples, which yield initial 87Sr/86Sr = 0.7038–0.7043 and eNd(t) = +3.4 to +6.9. Geochemical evidence suggests that the Sr Nd isotope differences reflect those in the mantle sources and are not due to crustal interactions, indicating that the sub-Gangdese batholithic mantle exhibited different degrees of enrichment at least ~50 Myr ago. These observations, combined with previous data, suggest that the sub-Gangdese batholithic mantle exhibited along-arc segmentation at 92°E–93°E at 50 Ma. This segmentation occurred probably between 90 and 47 Ma and was a result of the different characteristics and variable additions of the enriching agents into the asthenospheric mantle. CG gabbroic rocks were derived from depleted asthenospheric mantle enriched by variable amounts of aqueous fluids from the Neo-Tethyan slab, whereas EG rocks were generated from asthenospheric mantle enriched by hydrous melts from Indian continental crust. This interpretation implies that Indian continental crust was subducted beneath the southern Lhasa subterrane prior to 47 Ma. Compared with ~50 Ma rocks, EG ~30 Ma dykes exhibit higher melt-mobile element abundance and stronger REE fractionations, and were generated from deeper, melt-enriched mantle, signifying that a lithospheric thickening event occurred between 50 and 30 Ma in the EG batholith.

Potassic high Ba-Sr granitoids are widely distributed over collisional orogenic belts worldwide, however, their magma sources and petrogenetic processes are still debated. To better understand them, we collected a suite of Paleogene high Ba-Sr granitoids in the Jianchuan Basin, southeast Tibet. Based on the mineralogical compositions, major and trace element geochemistry, and age of formation, we subdivide them into two groups: intermediate series and granitic series. LA-MC-ICP-MS zircon U-Pb analyses show that the intermediate series crystallized at 33.8–36.9 Ma, similar to or slightly earlier than the granitic rocks with crystallization ages of 33.1–34.0 Ma. New geochemical and zircon Hf isotopic results, in combination with previous results, support an enriched mantle source for the high Ba-Sr intermediate rocks, and a mixed source of enriched mantle and lower crust for the granitic ones. We suggest that partial melting of enriched mantle with fractional crystallization of mafic minerals produced the potassic high Ba-Sr intermediate series. Meanwhile, the juvenile heat from such magma induced melting of thickened lower crustal materials which together with magma from mantle accounted for source of the granite series. The high Ba-Sr intermediate rocks have nearly identical major and trace element, and isotopic signatures as Archean sanukitoids, thus offer a potential constraint on the formation mechanism for the sanukitoids. Our study supports that the emergence of the sanukitoids requires a sediment-metasomatized mantle and thus indicates the onset of crustal recycling into the mantle.

Amphibolite from the core and the outer part of the Namche Barwa Complex (NBC), southeast Tibet, experienced upper amphibolite facies metamorphism and records clockwise metamorphic P-T paths. Prograde (M1), peak (M2) and retrograde (M3) metamorphic mineral assemblages were identified in both amphibolite types. The M2 P-T conditions in the core and outer part of the NBC are similar: Zhibai amphibolite (ZBA, from the core), ~750 °C/10 kbar; Paixiang amphibolite (PXA, from the outer part), 670–750 °C/ 8.5–10.5 kbar. SIMS zircon U-Pb dating suggests that the protolith of PXA (the outer part of the NBC) formed at ~90–80 Ma, similar to the mafic dykes within the Tethyan Himalaya. This magmatism is probably related to tectonic events on the northern Indian passive margin before its collision with the Eurasian Plate. Furthermore, zircon records metamorphism from Oligocene to late Miocene (~30 Ma to ~10 Ma) in the ZBA, while PXA only documents a Miocene metamorphic event (~9 Ma). It is inferred that from Oligocene to late Miocene, semi-continuous advection of relatively hot material from the depth might have operated only in NBC core, thus exhuming the ZBA to a shallower level, while this material did not efficiently reach/affect the PXA. At late Miocene (~10 Ma) rapid exhumation started to propagate to the outer part of NBC and both units were amalgamated as a coherent block.

The Himalayan orogen is featured by widespread S-type leucogranites (ca. 45 to 9 Ma) formed primarily from the partial melting of metapelites (800–480 Ma), and minor from the partial melting of amphibolite with subordinate metapelites. The Ama Drime gneiss and Mabja leucogranite pluton are both located at the footwall of the NS-trending Xainza–Dinggye normal fault in the central part of the Himalayan orogen, with published eNd(t) values of −21.0 to −19.6 and − 19.3 to −18.3 and 87Sr/86Sr(t) values of 0.90954–0.92574 and 0.84853–0.85474, respectively. In this study, we presented new SHRIMP zircon U-Pb ages and laser ablation-multi-collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) tourmaline boron isotope data on the metamorphic rocks and leucogranites of the Himalayan orogen. The weighted mean 207Pb/206Pb age of the zircon cores of the Ama Drime gneiss (T0446-2-3) is 1854 ± 4 Ma. The tourmaline δ11B values of the Ama Drime gneiss (T0446-1-6) are −17.6 to −14.3‰, similar to those of the Mabja leucogranite (T0436-4, −18.9 to −17.4‰). In contrast, the tourmaline δ11B values of the Quedang metapelite (T0389-18) and its partial melting product – Malashan leucogranite (T0659-12A-3) are substantially higher, i.e., −15.3 to −12.5‰ and − 16.2 to −8.0‰, respectively. The tourmaline δ11B values of the Yadoi leucogranite (T0321-4), derived from partial melting of amphibolite, range from −8.4 to −5.4‰. Therefore, the tourmaline B-isotopes and Sr-Nd-isotopes results are consistent, and may have reflected an E-W extension along the southern Tibetan rift system, indicating a late Miocene anatexis in the North Himalaya region. The melts derived from the partial melting of metapelites and mature crustal materials (e.g., Paleoproterozoic Ama Drime gneiss) ascended along the N-S trending Xainza-Dinggye normal fault, and were subsequently emplaced in the Tethyan Himalaya sequence.

Early Jurassic plutonic rocks from the Central Lhasa block that range from gabbro to granite yield similar 206Pb/238U ages ranging from 187.6 Ma to 196.0 Ma. Gabbros and diorites are characterized by similar radiogenic 87Sr/86Sr(t) ratios that range from 0.7053 to 0.7060, unradiogenic eNd(t) values ranging from −4.2 to −2.5 and eHf(t) values ranging from −8.8 to −2.8. Compared with the intermediate to mafic rocks, the granites have much higher 87Sr/86Sr(t) ratios, from 0.7102 to 0.7127, and lower eNd(t) values, from −8.7 to −7.5, and eHf(t) values from −11.9 to −7.5. Elemental and isotopic data suggest that the gabbros were derived from a melt-modified subarc mantle, diorites represent derivatives produced by hornblende fractional crystallization of gabbros, and granites were generated by the melting of ancient crustal basement rocks. Combined with literature data, these results suggest that the Central Lhasa block, as well as parts of the Southern and Northern Lhasa blocks, also contains ancient crystalline basement rocks.

Titanite, one of the major accessory phases in middle to lower crustal rocks, could play an important role in shaping trace element as well as radiogenic isotope compositions in amphibolite-derived melts. Petrographic, zircon U/Pb, whole-rock element and isotope (Sr and Nd) data on a suite of leucosomes, and titanite Nd isotope compositions from the migmatitic amphibolite complex, southern Liaoning, indicate that (1) the amphibolite with a protolith age of ~2450 Ma underwent low-degree partial melting at ~160 Ma and produced two groups of leucosome; (2) these leucosomes preserved a large magnitude of Nd isotope disequilibrium with up to ~10 epsilon units of difference with respect to their sources; (3) differential dissolution of titanite during partial melting might lead to negative correlations between Nd isotope compositions and respective Ti or V contents (proxy of the amount of titanite presented). These data indicates that during low-degree partial melting of mafic rocks, titanite stays as a residue phase due to either unfavorable texture or melting (temperature or fluid) conditions and leads to the development of melts with depletion in Ti-affinity elements, REE, and a large positive shift in Nd isotope compositions. Similar to other accessory phases (e.g., zircon, apatite, and monazite) in metasedimentary sources, this contribution presents another case that could be common in high-grade metamorphic terranes in which titanite could be a key phase that controls key trace element (e.g. Nb Ta, V, and REE) budget and Nd isotope compositions in melts derived from low-pressure melting of amphibolite or mafic rocks in various tectonic environments.

Syn-exhumation mafic magmatism during continental collision provides insights into the crust-mantle reaction during deep subduction and the nature of orogenic lithospheric mantle in collisional orogens. In this study, we present a comprehensive data set of zircon U-Pb ages and whole-rock major-trace elements as well as Sr-Nd-Pb isotopes of alkaline mafic rocks from the southern Liaodong Peninsula, eastern China. Zircon U-Pb analyses yield Late Triassic age of 213 ± 3 to 217 ± 3 Ma, younger than the Middle Triassic ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogen. Thus, the alkaline mafic rocks are products of syn-exhumation magmatism during continental collision of the South and North China blocks. The rocks show shoshonitic affinities with high K2O (3.78–5.23 wt%) and K2O/Na2O (0.71–1.22). They are characterized by arc-like trace-element patterns with enriched LILE, Pb, and LREE, and depleted HFSE. They exhibit enriched Sr-Nd isotopic compositions with high initial 87Sr/86Sr isotopic ratios of 0.7058–0.7061 and negative eNd(t) values of −13.0 to −15.1. These results suggest involvement of recycled continental crust in their mantle source. The mantle source likely formed by the metasomatic reaction of subducted continental crust-derived melts with the overlying subcontinental lithospheric mantle during the Triassic continental collision. Decompressional melting of this metasomatized mantle formed syn-exhumation mafic magmas during the transition from convergent to extensional tectonics in the Late Triassic. Accordingly, mafic rocks from the southern Liaodong Peninsula provide a geochemical record of the subduction and recycling of continental crust into the mantle and melt-mantle reaction induced metasomatism within the orogen.

In the Borong Gneiss Dome and Malashan Gneiss Dome within the Tethyan Himalaya, zircon U Pb ages and whole-rock geochemical data demonstrate that at least two episodes of leucogranite formed at 19.8–19.4 Ma and 18.6–18.5 Ma, respectively. Each of them contains at least two suites of granite: two-mica granite and garnet-bearing leucogranite. Although the two types of granites are characterized by distinct element geochemistry and mineral compositions, they show similar Sr-Nd-Hf isotope ratios and regular variations in the compositions of whole-rock elements. From two-mica granite to garnet-bearing leucogranite, SiO2, Na2O, Rb, Nb, and Ta contents and Rb/Sr ratios increase; in contrast, Al2O3, CaO, MgO, FeO, TiO2, Sr, Ba, Eu, Zr, U, Th, light rare earth elements (LREEs) and Sc contents, as well as Zr/Hf and Nb/Ta ratios, decrease. Such systematic variations imply that two-mica granite and garnet-bearing leucogranite are cogenetic and that two-mica granite represents the more primary melt, whereas garnet-bearing leucogranite is a more evolved melt. During granitic magma evolution, fractional crystallization induces substantial changes in the melt structure and in turn major changes in the dissolution behavior of accessory phases (e.g., zircon, monazite, and apatite) and the geochemistry of key trace elements. Such changes might be the key factors that resulted in the subordinate W-Sn-Nb-Ta-Be anomalies in the more evolved granites, which implies that the Himalayan Cenozoic leucogranites have high potential to produce economic rare metal deposits.

Recent studies in the Xiaru and Malashan gneiss dome of the Tethyan Himalaya, southern Tibet identify that Xiaru and Paiku tourmaline-bearing leucogranite dike formed at ~ 28–29 Ma. Together with ~ 28 Ma Kuday garnet-bearing leucogranites, it is demonstrated that the Himalayan orogenic belt experienced a major episode of crustal melting in the Mid-Oligocene. Geochemical data indicate that three suites of leucogranite are characterized by large variations in the major and trace element compositions as well as Sr–Nd isotope systematics, which could be explained by combined fractional crystallization and relative contributions of micas and accessory phases dissolved into a crustal melt during decompressional melting of metapelitic rocks. Documentation of Oligocene partial melting of crustal rocks could indicate that the exhumation of deep crustal rocks in the Himalayan orogenic belt could have started as early as Oligocene.