The origin of Alpine-Himalayan K-rich orogenic lavas: an integrated experimental and geochemical approach

发生在阿尔卑斯-喜马拉雅造山带内的幔源岩浆活动以富钾为主，其成因一直存在争议，特别是对大陆地壳中普遍存在的地球化学信号的意义存在争议。地壳成分主要表现为元素不兼容富集，87Sr/86Sr、207Pb/204Pb、187Os/188Os升高，143Nd/144Nd和176Hf/177Hf比值降低；ii)一种超贫成分，通常存在难熔cr尖晶石、高Fo橄榄石和低FeO丰度；(3) Th/La与Sm/La的高耦合表明其与杂岩具有遗传关系。上述观察结果表明，造山熔岩的来源不能真实地模拟为均质橄榄岩。认为交代组合位于不同生育力的橄榄岩围岩脉体中，形成于俯冲、碰撞和碰撞后三个主要造山阶段的熔融-地幔反应。为了模拟阿尔卑斯-喜马拉雅造山带岩浆地幔源内可能包含橄榄岩和非橄榄岩-含水矿物组合的熔融过程，我们将进行两种类型的实验：1)模拟俯冲过程中可能发生的不同大陆地壳物质的循环和再熔融；包括陆源硅质碎屑沉积物、泥质沉积物、碳酸泥岩和蓝脉岩，并结合不同富集度的橄榄岩，进行了高达3 GPa的夹层实验。ii)第二个系列实验将模拟与碰撞后阶段相关的融化事件，最终目标是产生成分类似阿尔卑斯-喜马拉雅造山熔岩的熔体。它将结合i)在第一系列实验中产生的含水组合，以及ii)设想包含造山熔岩来源的含水组合，如辉云母-斜辉石岩和微辉石岩（MARID），以及在高达3gpa的压力下不同生育能力的橄榄岩。通过进行这些实验，我们模拟了造山岩浆活动的主要阶段，在这个阶段中，第一次来自板块的含水沉积物融化渗透到上覆的地幔楔中，并经历了大规模的热和化学平衡（部分到完全的反应性冻结），第二次是由此产生的交代体的激活经历了造山地幔碰撞后的部分熔化。该研究的新颖性主要在于从橄榄岩熔炼到橄榄岩和非橄榄岩组合熔炼的范式转变。这揭示了我们对非橄榄岩超基性岩石融化的理解存在巨大差距，该项目将为填补这一空白迈出第一步。

Mantle-derived magmatism that occurs within Alpine-Himalayan orogenic belts is dominantly K-rich and its origin is controversial, particularly the significance of the widespread geochemical signal typical for recycled continental crust. Three major components are recognized: i) The crustal component indicated by incompatible-element enrichment, elevated 87Sr/86Sr, 207Pb/204Pb, 187Os/188Os and low 143Nd/144Nd and 176Hf/177Hf ratios of the lavas; ii) An ultra-depleted component identified by usual presence of refractory Cr-spinel, high Fo olivine and low FeO abundances; iii) Extremely high Th/La coupled with high Sm/La points to a genetic relationship with the melange. The above observations suggest that the source of orogenic lavas cannot be realistically modelled as homogeneous peridotite. It is envisaged that the metasomatic assemblages are situated in the veins within peridotitic wall-rock of different fertility, originated through melt-mantle reaction, during three major orogenic phases: subduction, collision and postcollision. In order to simulate the melting processes within mantle source of Alpine-Himalayan orogenic magmas, which may involve a mixture of peridotite and nonperidotite-hydrous mineral assemblages, we will perform two types of experiments: i) The first one mimic recycling and re-melting of different continental crustal material that may have happened during subduction; it involves sandwich experiments up to 3 GPa including terrigenous siliciclastic sediments, marly sediments, carbonated pelites and blueshists, combined with peridotite of different fertility. ii) The second series of experiments will simulate melting events related to postcollisional stage with ultimate goal to produce melts that compositionally resemble Alpine-Himalayan orogenic lavas. It will combine i) hydrous assemblages produced in the first series of experiments, and ii) hydrous assemblages envisioned to comprise source of the orogenic lavas like phlogopite-clinopyroxenites and glimerites (MARID), again with peridotite of different fertility at pressures up to 3 GPa. By performing these experiments, we simulate major stages involved in orogenic magmatism, where first hydrous sediment melt from the slab infiltrates the overlying mantle wedge and undergoes wholesale thermal and chemical equilibration (partial to complete reactive freezing), and the second when the activation of the resulting metasomes undergoes postcollisional partial melting in the orogenic mantle. The novelty of the proposed research lies largely in the paradigm shift from melting of peridotite to the melting of combined peridotitic and non-peridotitic assemblages. This reveals immense gaps in our understanding of the melting of non-peridotitic ultramafic rocks that this project will make the first steps in filling.