Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane, south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting
Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane, south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting
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DOI：
10.1016/j.gr.2017.02.008
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发表时间：
2017-06
影响因子：
6.1
通讯作者：
Zhihui Cheng;Zhengfu Guo
中科院分区：
文献类型：
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作者：
Zhihui Cheng;Zhengfu Guo
藏南拉萨地区赛里普火山岩区广泛分布着含幔源捕虏体的碰撞后超钾质岩浆岩（15.2- 18.8Ma）。根据岩石学和地球化学特征，可将其划分为高MgO亚类和低MgO亚类。高MgO亚组的斑晶物相为橄榄石-I（Fo 87 -92）、金云母和单斜辉石，而低MgO亚组的斑晶物相主要为金云母、单斜辉石和橄榄石-II（Fo 77 -89）。这些超钾质岩浆岩具有较高的MgO（4.6- 14.5wt%）、Ni（145-346 ppm）、Cr（289-610 ppm）含量，在原始地幔标准化微量元素图解中，轻稀土元素（REE）相对于重稀土元素（REE）富集，大离子亲石元素（LILE）相对于高场强元素（HFSE）富集，Nb-Ta-Ti具有强烈的负异常。它们有极强的放射性（87锶/86锶）i（0.7167-0.7274）和非放射成因（143 Nd/144 Nd）i（0.5118-0.5120），在（206 Pb/204 Pb）i（18.363-18.790）范围内，（207 Pb/204 Pb）i（15.740-15.816）和（208 Pb/204 Pb）i（39.661-39.827）较高，δ 18 O值较高（7.3-9.7‰）。亏损洋中脊玄武岩源地幔（DMM）与印度陆壳（HHCS）在Sr-Nd-Pb-O同位素图解上的强线性相关性表明，其地球化学特征可能是地幔橄榄岩与榴辉岩化的印度陆壳（HHCS）在地幔楔中释放的富集组分（流体和熔体）反应的结果。高MgO（13.7- 14.5wt%）亚组比低MgO（4.6- 8.8wt%）亚组具有更高的（143 Nd/144 Nd）i、更低的（87 Sr/86 Sr）i和（206 Pb/204 Pb）i比值以及更低的δ 18 O值。橄榄石斑晶的高Ni含量（850-4862 ppm）和全岩高SiO2、NiO、低CaO含量，表明低MgO超钾质岩浆岩是贫橄榄石地幔辉石岩部分熔融的产物。而高镁超钾质岩石中橄榄石斑晶Ni含量较低，全岩SiO2、NiO含量较低，CaO含量较高，可能是橄榄岩地幔源区的指示。这可能是由于印度大陆岩石圈向北俯冲造成的地幔楔中的母岩浆地幔源中加入了不同量的富硅酸盐组分。首次报道的反应形成的橄榄岩捕虏体，主要由单斜辉石（45- 65vol%）和斜方辉石（30- 50vol%）组成，并含有少量的金云母（<3vol%）和石英（<2vol%）。方辉橄榄岩是藏南地幔捕虏体的另一种主要类型，其矿物组合为橄榄石（60- 75vol%）、斜方辉石（20- 35vol%）、单斜辉石（<3vol%）、金云母（<2vol%）和尖晶石（<2vol%），可能经历了俯冲交代作用。结合两类超钾质岩浆，我们认为藏南地幔楔的成分可能是从辉橄榄岩经二辉橄榄岩逐渐演化为二辉橄榄岩，其源区富硅酸盐组分交代作用强烈。25-8 Ma期间，印度大陆板片的回滚可能触发了藏南富集地幔源区的部分熔融。
Post-collisional ultrapotassic magmatic rocks (15.2–18.8 Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5 wt%), Ni (145–346 ppm), Cr (289–610 ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (87Sr/86Sr)i(0.7167–0.7274) and unradiogenic (143Nd/144Nd)i(0.5118–0.5120), high (207Pb/204Pb)i(15.740–15.816) and (208Pb/204Pb)i(39.661–39.827) at a given (206Pb/204Pb)i(18.363–18.790) with high δ18O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5 wt%) subgroup displays higher (143Nd/144Nd)i, lower (87Sr/86Sr)iand (206Pb/204Pb)iratios and lower δ18O values compared with the low-MgO (4.6–8.8 wt%) subgroup. High Ni (850–4862 ppm) contents of olivine phenocrysts and high whole-rock SiO2, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO2, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65 vol%) and orthopyroxene (30–50 vol%) with minor phlogopite (< 3 vol%) and quartz (< 2 vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75 vol%), orthopyroxene (20–35 vol%), clinopyroxene (< 3 vol%), phlogopite (< 2 vol%) and spinel (< 2 vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8 Ma in south Tibet.