Abstract Mantle-derived magmas commonly undergo complex evolutionary processes (e.g., magma mixing/recharge, crystal-mush reaction, magma differentiation, crustal contamination) during their ascent. These processes can be difficult to discriminate when looking at whole-rock composition for hybrid rocks. However, minerals preserve textural and chemical signatures that are critical for gaining new insights into the magma plumbing system of these hybrid rocks. Melilitites, which are typical silica-undersaturated, alkali- and carbon-rich rocks. It remains unclear whether they have experienced complex magma processes. In this study, we focus on clinopyroxene (Cpx) phenocrysts from the Haoti Cenozoic melilitites, from West Qinling (WQ), northeastern Tibetan Plateau, and four types have been identified. Both type-1 and type-2 are in equilibrium with whole-rock compositions and can be classified as magmatic autocrysts. Type-1 Cpx features wider clean core and narrow rim. Type-2 is characterized by sieve-textured cores and patchy zoning, recording subsequent melt modification. Type-3 also displays sieved texture with primitive core compositions (Mg# = 87.8–92.2, Cr2O3 = 1.25–2.00 wt.%) overlapping the Cpx from mantle xenoliths, indicating mantle-derived xenocrysts associated with subsequent melt modification. Type-4 shows core-mantle-rim complex zoning and displays negative Eu and Sr anomalies and low Mg# values (67.1–70.3) in the core, suggesting its xenocrysts originate from lower crust feldspar-rich granulite. The oscillatory zoning and abrupt Cr2O3 decrease (from ~0.8 wt.% to ~0.2 wt.%) in the mantle testifies evolved magma recharge events. All Cpxs display narrow overgrowth rims characterized by depletions in Mg and Si but enrichments in Al and Ti, reflecting degassing and rapid crystallization during pre-eruptive magma decompression. The olivine phenocrysts comprise both magmatic autocrysts (Fo = 85.9–87.0, average value = 86.5) and mantle-derived xenocrysts (Fo = 84.8–92.3, average value = 90.4). The quantitative calculations reveal that high- and low-Cr Cpx domains crystallized under 1132–1176 °C/5.6–7.9 kbar and 1101–1158 °C/3.6–6.7 kbar, respectively. Cpx hygrometry reveals that both high- and low-Cr magma batches are hydrous, with H2O contents of ~3.5 wt.% and ~2.7 wt.%. Empirical barometric results reveal that Cpx rims record significantly lower crystallization pressures than their core/mantle domains. Consequently, three discrete magma reservoirs have been revealed at depths of ~26.2 km, ~21.1 km, and shallower levels. Cpx phenocrysts exhibit homogeneous 87Sr/86Sr ratios ranging from 0.7040 to 0.7046. The sieved domains of type-2 Cpx exhibit the lower 87Sr/86Sr ratios compared to those of type-3 Cpx, indicating modification by late-stage melts derived from distinct sources. The melt compositions in equilibrium with Cpx autocrysts display high La/Yb (71.98–111.22), Dy/Yb (3.31–4.67), and Zr/Hf (47.29–55.62) ratios, but low Hf/Sm (0.55–0.87) and Ti/Eu (4939–5960) ratios, indicating a carbonate-metasomatized garnet stability field mantle source. The olivine autocrysts exhibit low NiO contents (0.04–0.29 wt.%), low Ni/(Mg/Fe)1000 ratios (0.9), and high 100×Mn/Fe ratios (up to 2.2), indicating a peridotitic source lithology. The diverse textures and compositional variations in Cpx attest to recurrent melt influx events capable of perturbing the thermal, baric, and volumetric equilibria between the quiescent magma reservoir and surrounding rocks. This study highlights melt influx-triggered melt hybridization that could be a key mechanism of activating magma reservoirs and triggering the Haoti volcano eruption during the outward growth of the Tibetan Plateau.
Xia et al. (Fri,) studied this question.