Guangying Feng

Origin of lamprophyres from the northern margin of the North China Craton: implications for mantle metasomatism

Abundant lamprophyre dykes occur near the northern margin of the North China Craton and offer a unique opportunity to study the nature of the mantle source. The dykes are minettes composed of phlogopite, sanidine and calcite. 40Ar/39Ar dating yields ages of 234 ± 2 and 222 ± 6 Ma. The lamprophyres are near-primary, mantle-derived ultrapotassic melts, having low SiO2 (31.0 – 41.5 wt%) and high K2O (4.40 – 7.12 wt%) contents, high Mg# (62 – 84) and high contents of compatible elements. They are characterized by fractionated rare earth element patterns, radiogenic Sr and unradiogenic Nd isotope compositions (87Sr/86Sri = 0.7070 – 0.7075; εNd(t) = −12.8 to −9.2). A small amount of mafic crustal rocks (<4.4%) may have been assimilated during magma ascent, as revealed by 187Os/188Osi ratios of 0.4548 – 0.8068. These data suggest that the lamprophyres originated from a low degree of partial melting of an enriched subcontinental lithospheric mantle source with abundant phlogopite, clinopyroxene and carbonate. The source has been metasomatized by carbonate- and potassium-rich fluids derived from carbonated sediments recycled via subduction of Palaeo-Mongolian oceanic slab beneath the North China Craton. Supplementary material: Microprobe analyses of biotite, feldspar and carbonate, 40Ar–39Ar analytical data, and 36Ar/40Ar v. 39Ar/40Ar inverse isochron diagrams for phlogopite phenocrysts and groundmass from the Datong lamprophyres are available at https://doi.org/10.6084/m9.figshare.c.3574265.

Petrological and Re-Os isotopic constraints on the origin and tectonic setting of the Cuobuzha peridotite, Yarlung Zangbo suture zone, southwest Tibet, China

The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the Yarlung Zangbo suture zone in southwest Tibet comprises mainly clinopyroxene (cpx)-rich and depleted harzburgites. Spinels in the cpx-harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx-harzburgites have suprachondritic 187 Os/ 188 Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx-harzburgites formed from low-degree partial melting of a primitive mantle source, and they were subsequently modified by melt-rock interactions in a mid-oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx-harzburgites, which later interacted with mid-oceanic ridge basalt– or island-arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo-Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Petrological and Re-Os Isotopic Constraints on the Origin and Tectonic Setting of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, SW Tibet, China

The Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north (Dupuis et al., 2005; Yang et al., 2011). In the western part of the YZSZ, the Northern and the Southern sub-belts form two sub-parallel zones of mafic – ultramafic rock assemblages with overlapping crystallization ages (Xiong et al., 2011; Hebért et al., 2012; Liu et al., 2015). The upper mantle section of the Cuobuzha ophiolite in the northern sub-belt of the Yarlung–Zangbo Suture Zone (YZSZ) in SW Tibet comprises mainly clinopyroxene (cpx)–rich and depleted harzburgites. Spinels in the cpx-harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx-harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380-0.575 ppb). The cpx-harzburgites plot in a Re vs. Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid-ocean ridge melt evolution processes (Uysal et al., 2015). Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-ocean ridge type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx-harzburgites formed from low-degree partial melting (~5%) of a primitive mantle source, and they were

Kalaymyo Peridotite Massif in the Indo-Myanmar Ranges (Western Myanmar): Its Mineralogy and Petrology

Mesozoic ophiolites crop out discontinuously in the Indo-Myanmar Ranges in NE India and Myanmar, and represent the remnants of the Neotethyan oceanic lithosphere (Sengupta et al., 1990; Mitchell, 1993). These ophiolites in the Indo-Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet farther northwest (Mitchell, 1993; Fareeduddin and Dilek, 2015), as indicated by their coeval crystallization ages and geochemical compositions (Yang et al., 2012; Liu et al., 2016). The Kalaymyo ophiolite is located in the central part of the eastern Indo-Myanmar Ranges (Fig. 1). composition of these ophiolites from the central Tibetan Plateau (CTP) is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs, which is inconsistent with most ophiolites worldwide (Robinson and Zhou, 2008; Zhang et al., 2008). But the generation and tectonic nature of these ophiolites are still controversial. * The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures. They are composed of olivine (Fo = 89.8–90.5), orthopyroxene (En86-91Wo1-4Fs8-10; Mg# = 89.6–91.9), clinopyroxene (En46-49Wo47-50Fs3-5; Mg# = 90.9–93.6) and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1–90.8 and SiO2 (41.5–43.65 wt.%), Al2O3 (1.66–2.66 wt.%) and CaO (1.45–2.67 wt.%) contents.

Ocean-continent Transition to Suprasubduction Zone Origin of the Western Yarlung Zangbo Ophiolites in SW Tibet, China: Multi-stage, Transient Evolution of the Neotethyan Oceanic Lithosphere

The ophiolites that crop out discontinuously along the ~2000 km Yarlung Zangbo Suture zone (YZSZ) between the Nanga Parbat and Namche Barwa syntaxes in southern Tibet represent the remnants of Neotethyan oceanic lithosphere (Fig. 1a). We have investigated the internal structure and the geochemical makeup of mafic-ultramafic rock assemblages that are exposed in the westernmost segment of the YZSZ where the suture zone architecture displays two distinct sub-belts of ophiolitic and mélange units separated by a continental Zhongba terrane (Fig. 1b). These two sub-belts include the Daba – Xiugugabu in the south (Southern sub-belt, SSB) and the Dajiweng – Saga in the north (Northern sub-belt, NSB). We present new structural, geochemical, geochronological data from upper mantle peridotites and mafic dike intrusions occurring in these two sub-belts and discuss their tectonomagmatic origin. In-situ analysis of zircon grains obtained from mafic dikes within the Baer, Cuobuzha and Jianabeng massifs in the NSB, and within the Dongbo, Purang, Xiugugabu, Zhaga and Zhongba in the SSB have yielded crystallization ages ranging between130 and 122 Ma. Dike rocks in both sub-belts show N-MORB REE patterns and negative Nb, Ta and Ti anomalies, reminiscent of those documented from SSZ ophiolites. Harzburgitic host rocks of the mafic dike intrusions mainly display geochemical compositions of abyssal peridotites (Fig. 2), with the exception of the Dajiweng harzburgites, which show the geochemical signatures of forearc peridotites (Lian et al., 2016). Extrusive rocks that are spatially associated with these peridotite massifs in both sub-belts also have varying compositional and geochemical features. Tithonian to Valanginian (150 – 135 Ma) basaltic rocks in the Dongbo massif have OIB-like geochemistry and 138 Ma basaltic lavas in the Purang massif have EMORB-like geochemistry (Liu et al., 2015). Tuffaceous rocks in the Dajiweng massif are 140 Ma in age and show OIB-like geochemistry. We interpret these age and geochemical data to reflect a rifted continental margin origin of the extrusive rock units in both sub-belts. These data and structural observations show that the western Yarluang Zangbo ophiolites represent fragments of an Ocean-Continent Transition (OCT) peridotites altered by fluids in an initial supersubduction setting. We infer that mafic-ultramafic rock assemblages exposed in the SSB and NSB initially formed in an ocean – continent transition zone (OCTZ) during the late Jurassic, and that they were subsequently emplaced in the forearc setting of an intraoceanic subduction zone within a Neotethyan seaway during 130 to 122 Ma. The NSB and SSB are hence part of a single, S-directed nappe sheet derived from a Neotethyan seaway located north of the Zhongba terrane. *