Yan Xia

Crystal-melt separation and cumulate remelting within subvolcanic silicic magma reservoirs are of great significance for deciphering catastrophic volcanic eruptions, the generation of highly fractionated granites/rhyolites, and volcanic-plutonic relationships. However, the effective identification of crystal-melt separation and cumulate remelting is a matter of approach. To address this, we present a substantial number of feldspar in-situ major and trace element analyses, complemented by detailed microstructural observation, TIMA, whole-rock geochemical analyses, and zircon U-Pb dating and Hf isotope analyses. Our focus is on the Late Cretaceous Daiyunshan volcanic field, SE China, which includes the Pucheng quartz monzodiorite and the Shiniushan and Yunshan volcanic-plutonic complexes. We propose that the Pucheng quartz monzodiorite represents the intermediate magma reservoir of the Shiniushan and Yunshan volcanic-plutonic complexes. The intermediate magma reservoir evolved further into the Yunshan subvolcanic silicic mush through crystal-melt separation dominated by plagioclase and into the Shiniushan subvolcanic silicic mush through crystal-melt separation dominated by both alkali feldspar and plagioclase, respectively. Alkali feldspar is usually the dominant mineral involved in mush processes within silicic magma reservoirs. Therefore, Ba and Sr variation trends, as well as Ti, Mg, Li, Eu, Gd, and light rare earth element characteristics of alkali feldspar from different rock types within a rock series, were used as indicators of crystal-melt separation within the intermediate-felsic mush. For the Shiniushan volcanic-plutonic complex, typical remobilization and cumulate remelting induced by magma recharge occurred within the subvolcanic silicic mush after crystal-melt separation, which is reflected by typical and pervasive melt resorption textures of plagioclase, alkali feldspar, quartz, and mafic minerals. The Ba content in the interstitial matrix alkali feldspar and in the recrystallized rims of the alkali feldspar phenocrysts increased to significantly higher values than the core values during the alkali feldspar-dominated cumulate remelting (the same trend also occurs for Gd due to amphibole remelting). This remelting process caused an “inverse Ba zoning” pattern in the alkali feldspar phenocrysts. The mush remobilization and cumulate remelting further induced the extrusion of crystal-rich rhyolite. Moreover, the cumulate remelting reduced the phenocryst content (i.e., increased porosity) of the porphyritic quartz monzonite formed by residual mush, thereby obscuring its cumulate framework.

东南沿海晚中生代幕式岩浆作用是解析古太平洋板块俯冲动力学的关键窗口，相较于浙闽沿海，对粤东岩浆作用的研究仍较为薄弱。本文对粤东莲花山断裂带周边花岗岩及其中的暗色包体开展了详细的岩相学、锆石U-Pb定年、全岩主微量元素及Sr-Nd-Hf-O同位素研究，结合已有相关定年资料，厘定了粤东晚中生代的四期岩浆作用：早侏罗世（~192-180 Ma）、中晚侏罗世（~172-150 Ma）、早白垩世早期（~148-130 Ma）及晚期（~106-94 Ma）。这些花岗岩的地球化学组成变化受控于岩浆的结晶分异作用，其中早侏罗世与早白垩世暗色包体与寄主岩的Sr-Nd-Hf同位素组成一致，为同源包体，而非岩浆混合作用的产物。早侏罗世闪长玢岩低硅、高铝、富钾、准铝质-弱过铝质，右倾稀土配分（Eu弱负异常），δ18O 值为 5.1‰~5.4‰，亏损ɛHf(t)（+8.52~+11.06），具典型弧岩浆特征，指示古太平洋板块俯冲于东南沿海之下应早于180 Ma，且四期幕式岩浆活动揭示俯冲过程呈现活动-停滞交替的间歇性特征。

Highly differentiated magmas are closely related to the formation of tin deposits. The Lianhuashan Basin and Baiyunzhang Basin, surrounded by multiple coeval tin deposits, developed various types of volcanic and subvolcanic rocks. The diagenetic connections between different types of rocks within the two basins provide new insight into the evolution of the silicic magma systems and the impact of highly differentiated magmas on tin mineralization. Both basins have consistent zircon U–Pb ages (143–138 Ma) and similar whole-rock Nd isotopes ( ε _Nd ( t ) = −5.7 to −3.4) and zircon Hf–O isotope ranges ( ε _Hf ( t ) = −9.0 to −2.5; δ ¹⁸ O = 5.1–7.9), suggesting that both basins originate from the same deep-level magma reservoir, followed by different degrees of crystal differentiation and several episodes of crystal–melt separation in their respective shallow-level magma reservoirs. The multiple pulses of magma extraction eventually produced different volcanic rocks (granite porphyry and rhyolite porphyry), whereas the remaining crystal mush consolidated in situ to form quartz monzonite porphyry. Further studies show that the tectonic regime changed from a compressive to an extensional environment at c . 140 Ma. Consequently, mantle-derived magmas with low oxygen fugacity injected shallow magma reservoirs that were able to evolve to a high degree of differentiation through multiple recharge, thus favouring the formation of the Sn deposit.

The extensive magmatism along the southeastern coast of China during Late Cretaceous time formed a volcanic-intrusive complexes belt, on which many volcanic basins and calderas are located. A growing amount of research has been carried out to reveal the formation processes of the Cretaceous volcanic-intrusive complexes in SE China, especially the volcanic-plutonic complexes in Zhejiang and Fujian Provinces. However, volcanic-plutonic complexes in Guangdong Province have not received enough attention in the past relative to volcanic-plutonic complexes in Zhejiang and Fujian Provinces. Furthermore, it is difficult to identify the genetic connections and formation processes of different type of volcanic rock with its co-genetic intrusive rocks in volcanic basins or calderas with complex igneous rock types, but it also provides new insights into the volcanic plumbing system processes and magma reservoir differentiation. Therefore, we conducted detailed research on the volcanic-plutonic complexes in the Lianhuashan Basin and Baiyunzhang Basin in the Lianhuashan Fault Zone. The Lianhuashan volcanic-plutonic complex is comprised of ignimbrite, granite porphyry and rhyolite porphyry, while the Baiyunzhang volcanic-plutonic complex is comprised of ignimbrite, trachyte, rhyolite and quartz monzonite porphyry. The LA-ICP‒MS zircon U‒Pb dating results indicate that the crystallization of volcanic and intrusive rocks was contemporaneous within the error (143~138 Ma), and they had similar ranges of zircon Hf-O isotopes (εHf(t)= -2.8 to 8.9; δ18O=5.53 to 7.87 ‰) and whole-rock Nd isotopes (εNd(t)=-3.4 to -5.7). In addition, samples with high SiO2 contents (SiO2 ≥ 70 wt.%) exhibit high Rb/Sr ratios and depletions in Ba, Sr, P, Ti and Eu, while the samples with low SiO2 contents exhibit low Rb/Sr ratios and weak depletions in Ba, Sr, P, and Ti with insignificant negative Eu anomalies. Therefore, we suggest that the parent magmas of the Baiyunzhang-Lianhuashan Basins originated from the same deep-level magma reservoir, which underwent pulsatile magma extraction and formed two shallow-level magma reservoirs. The two shallow magma reservoirs each underwent different degrees of crystallization differentiation and crystal-melt separation, eventually forming ignimbrite, trachyte, rhyolite, granite porphyry and rhyolite porphyry through pulsed extraction. The remaining crystal mush consolidated in situ to form quartz monzonite porphyry intrusive rocks.

Many studies have revealed that mantle‐derived mafic rocks can be more isotopically enriched than coexisting crustal felsic rocks. However, its origin remains to be elucidated. Here, we report on some Early Cretaceous (ca. 130 Ma) mafic rocks in Gan‐Hang Belt (South China) that are more isotopically enriched than the temporally and spatially associated granitoids. Relatively depleted Nd‐Hf isotopic signatures [ε_Nd(t) = −0.4 to +0.6, ε_Hf(t) = 3.4 to 4.8, ε_Hf(t)_zircon = −2.3 to +9.3], coupled with low zircon δ¹⁸O_SMOW values (5.33–6.84 ‰) and geographical proximity to the exposed island arc rocks, indicate that the isotopically depleted Daixi alkali feldspar granites were dominantly derived from the relatively young Neoproterozoic accreted Shuangxiwu island arc sequences. Yet new and published whole‐rock Sr‐Nd‐Hf‐Pb‐O isotopic data [⁸⁷Sr/⁸⁶Sr)_i = 0.70788–0.70833; ε_Nd(t) = −7.6 to −3.5; ε_Hf(t) = −6.8 to −1.9; (²⁰⁶Pb/²⁰⁴Pb)_i = 18.028–18.108, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.565–15.577, (²⁰⁸Pb/²⁰⁴Pb)_i = 38.198–38.341; δ¹⁸O_SMOW = 4.5–5.5 ‰] highlight that the isotopically enriched Siling hornblende gabbros were predominantly sourced from an older, metasomatically enriched SCLM of adjacent Yangtze cratonic affinity. The arc crustal allochthon is assumed to have been tectonically emplaced onto the southeast Yangtze cratonic foreland, resulting in isotope inversion with younger crust on top of the older lithospheric mantle. Our results reveal a juxtaposition geometry of continental material after collision and accretion of island arcs by using isotope methods, which is independently supported by tectonic geological and geophysical studies. It suggests that ancient lithospheres should exist at depth within certain orogens associated with island arc accretion.

The origin of the Cathaysia block and its position in Columbia supercontinent are still enigmatic. Paleoproterozoic magmatisms developed in the Cathaysia block due to two episodes of continental back‐arc extension range in ages of circa 1.88–1.85 and 1.78–1.77 Ga. A long‐lived ultradepleted mantle domain with highly incompatible element depletion related to average depleted mantle once underlaid the Cathaysia block is revealed by the highly depleted Hf‐Nd isotopes of the Paleoproterozoic meta‐mafic rocks. A self‐consistent set of geological, geochronological, and geochemical data indicate that the Cathaysia was assembled and then maintained a position off the southern margin of North Australia Craton from the assembly to the accretion of Columbia supercontinent (circa 1.90 to 1.74 Ga). Given the evidence for Cathaysia block, mantle heterogeneity might provide an alternative approach of supercontinent reconstruction.