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Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure-temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750-830 °C, and recorded three metamorphic stages at kyanite (M), sillimanite (M) and cordierite-spinel grade (M). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U-Th-Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500-400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M-stage (~30-29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M-stage, 21-19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure-temperature-time data constrains peak high-pressure metamorphism within the GHC to ~30-29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks. [ABSTRACT FROM AUTHOR]

[Display omitted] • Accessory minerals geochemistry characteristics are affected by prior or cocrystallizing minerals. • The Cl contents in melts are important for forming porphyry Cu-Au deposits. • Zircon and apatite geochemistry combination is useful for refining targets in the SLT. The products of long-lasting magmatism (ca. 170 Myr) related to the northward subduction of the Neo-Tethys Ocean are preserved in the southern Lhasa subterrane (SLT). However, only a few large porphyry Cu–Au deposits related to these igneous rocks have been discovered in the Xiongcun ore concentration area. Here, we present zircon and apatite data on Jurassic fertile suites (FS), barren suites (BS), and recently proposed prospecting target suites (PTS) in the SLT, intending to present a case study of the regional-scale porphyry metallogenic potential. The zircon geochemical data with Rayleigh fractionation modeling show that the zircon geochemical characteristics of the FS and PTS were affected by high proportions of amphibole and titanite crystallization, whereas the zircon geochemistry of the BS displays the effects of plagioclase fractionation or mixing with the least evolved melts. The zircon Ce4+/Ce3+ and Eu/Eu* values cannot be used to trace the oxygen fugacity of the magma due to prior or simultaneous mineral crystallization. However, the zircon Ce/Ce* and Ce/Nd ratios may be used as semiquantitative measures of the magma redox conditions. This result is similar to that calculated by the new magmatic oxybarometer using trace elements in zircon, showing that the FS (ΔFMQ = 1.21 ± 0.42) and PTS (ΔFMQ = 1.29 ± 0.51) have higher oxygen fugacity levels than the BS (ΔFMQ = 0.40 ± 0.88). The melt S contents estimated from magmatic apatites are lower in the FS and PTS than in the BS, which is attributed to prior anhydrite crystallization under high oxygen fugacity and low temperature conditions. The higher Cl in the melt of the FS (average 0.098 wt%) than in the melts of the PTS (average 0.045 wt%) and BS (average 0.04 wt%) shows that elevated Cl contents in melts are likely important for forming porphyry Cu–Au deposits. The combined geochemical characteristics of zircon and apatite suggest that a high oxygen fugacity (ΔFMQ > 1.2), prior or concurrent anhydrite crystallization, and high zircon Eu/Eu* (>0.6), Ce4+/Ce3+, and (Yb/Dy) N values can be used to refine potential porphyry Cu–Au targets on a regional scale in the SLT. High melt Cl contents and apatite MnO and Pb contents can effectively indicate fertile suites at the deposit scale. [ABSTRACT FROM AUTHOR]

The Lhunze region of southeast Tibet uniquely juxtaposes Cenozoic granitic plutons with subvolcanic rocks in the Tethyan Himalayan Sequence. In this study, the potential petrogenetic connection between two-mica granite and subvolcanic rhyolite porphyry from the Lhunze area is investigated using petrology, geochronology, and geochemistry. Monazite Th Pb ages imply the two-mica granite crystallized during the mid-Eocene at c. 43.5–43.1 Ma, consistent with previous zircon U Pb isotope data. Monazite in the porphyry also records Th Pb ages of c. 44.5–42.9 Ma , suggesting contemporaneous emplacement with the two-mica granite. Porphyritic structure, feldspar aggregates and comb alignment demonstrate that the two-mica granite resulted from transient melt flow associated with melt extraction. Some plagioclase phenocrysts from the porphyry have consistent An contents with the two-mica granite, indicating that both the two-mica granite and the porphyry probably originated from a similar melt. However, the porphyry has a higher SiO 2 concentration (72.2–75.9 wt%) than the two-mica granite (69.0–73.0 wt%). In addition, the porphyry is characterized by lower TiO 2 , MgO, CaO, Ba, Sr, and Zr contents as well as Zr/Hf, Sr/Y, Nb/Ta ratios than the two-mica granite. Moreover, strontium and REE contents of monazite in both the two-mica granite and porphyry also imply that crystal fractionation dominated melt evolution. Whole-rock Sr Nd and in situ monazite Nd isotope compositions indicate that the porphyry is slightly more radiogenic than the two-mica granite, implying that the porphyry likely assimilated more ancient country rocks during its evolution. We interpret the two-mica granite and porphyry as comagmatic intrusives linked by fractional crystallization of a cognate magma mush, in which the crystals accumulated to form the two-mica granite, and the complementary high silicic melt segregated to form the porphyry. Plagioclase accumulation is responsible for the generation of the high Sr /Y adakitic geochemical feather within two-mica granite. The scenario of complementary melt extraction and crystal fractionation might also have been an important process in forming the Himalayan leucogranites. Regionally, Neo-Tethyan slab breakoff during the Eocene could have increased mantle heat flow and triggered the generation and extraction of large volumes of silicic melt. • Two-mica granite and subvolcanic rhyolite porphyry have a coeval crystallization age at 44–43 Ma. • Two-mica granite represents cumulate and the porphyry as its complementary highly fractionated melt. • Magma differentiation via melt extraction from crystal mush is an effective mechanism to produce Himalayan leucogranites. [ABSTRACT FROM AUTHOR]

Determining the geochronological framework and geochemical nature of magmatism associated with orogenic processes is crucial to decipher the evolution of orogenic belts. The Himalayan orogen has recorded considerable information regarding deformation, metamorphism, and magmatism related to orogenic processes. Here, we identify four stages of granitic magmatism on the basis of monazite U–Th–Pb dating of rocks from the Lhagoi Kangri dome, north Himalayan antiform, southern Tibet: middle Eocene (42–40 Ma), late Eocene (ca. 36 Ma), Oligocene (ca. 26 Ma), and Miocene (16–13 Ma). The middle Eocene tourmaline-bearing leucogranite is characterized by (1) relatively high SiO 2 (78.7 wt%), Al 2 O 3 (12.6 wt%), and Na 2 O (6.3 wt%) contents; (2) enrichment in light rare earth elements (LREEs), depletion in heavy REEs, high abundances of REEs, and pronounced negative Eu anomalies; and (3) relatively low values of whole-rock Sr Nd isotope compositions (87Sr/86Sr(t) = 0.72101 and εNd(t) = −9.8). And the coeval garnet–bearing two–mica granite has a pronounced negative Eu anomaly (Eu/Eu* = 0.07) and a significate lanthanide tetrad effect (TE 1,3 = 1.46). These geochemical data suggest that these leucogranites underwent fractional crystallization. They provide a robust evidence for that anatexis of the lower crust of Indian plate began at 42–40 Ma in the central Himalayan orogen. The Oligocene and Miocene two–mica granites share broadly similar geochemical characteristics, such as moderate REE contents and weak negative Eu anomalies, and are distinct from the garnet-bearing muscovite leucogranite. Whole-rock Sr Nd isotope compositions of the Oligocene and Miocene leucogranites differ from those of the middle Eocene leucogranites, with higher 87Sr/86Sr(t) ratios (0.73524–0.74188) and lower εNd(t) values (−14.7 to −13.7). These geochemical data indicate that the Oligocene and Miocene leucogranites are products of muscovite-dehydration melting of metasedimentary rocks. The garnet-bearing muscovite leucogranite shows the REE tetrad effect, which suggests that it is more evolved than the coeval two–mica granite. We therefore infer that the variations in age and isotopic characteristics of the studied leucogranites were caused by a change in melt source resulting from the compression–extension evolution of the Himalayan collisional orogenic system. • Four-stage emplacement of leucogranites were identified in the Lhagoi Kangri dome. • The age and isotopic variations of leucogranites result from a change in melt source. • Middle Eocene leucogranites reveal a long-lived anatexis in the Himalayan orogen. [ABSTRACT FROM AUTHOR]