UHP metamorphism documented in Ti-chondrodite- and Ti-clinohumite-bearing serpentinized ultramafic rocks from Chinese southwestern Tianshan
UHP metamorphism documented in Ti-chondrodite- and Ti-clinohumite-bearing serpentinized ultramafic rocks from Chinese southwestern Tianshan
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中国西南天山含钛球粒陨石和钛斜腐铁矿的蛇纹石化超镁铁质岩石中记录的超高压变质作用
DOI：
10.1093/petrology/egv042
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发表时间：
2015
影响因子：
3.9
通讯作者：
Zhang Lifei
中科院分区：
文献类型：
--
作者：
Shen Tingting;Hermann Jőrg;Zhang Lifei
中国西南天山变质带记录了高压（HP）至超高压（UHP）条件，对应于寒冷的大洋俯冲带背景。蛇纹岩包围退变质榴辉岩和杆柱榴辉岩，在超高压单元的变岩中以透镜体的形式出现，超高压单元中还含有柯石英榴辉岩。根据这些蛇纹岩的岩石学和岩相学特征，可将其划分为5个事件：（1）地幔中的韦氏体-方辉橄榄体-纯橄榄体组合的形成，（2）地幔岩石在靠近海底的地方折返过程中的退变质作用和部分水化作用，（3）大洋变质作用导致的第一次蛇纹岩化和杆柱岩化作用，（4）俯冲过程中的超高压变质作用，（5）地幔岩石的退变质作用和部分水化作用。（5）折返过程中的退变质作用和第二次蛇纹岩化作用。蛇纹化伟氏岩的峰期变质矿物组合为Ti-软骨石+橄榄石+叶蛇纹石+磁铁矿+水镁石。一个计算的假截面，这蛇纹状韦氏岩表明，叶蛇纹石中的铝含量是最敏感的温度，但也可以用来约束压力。叶蛇纹石的平均XAl = 0.204 ± 0.026（XAl= Al（a.p.f.u.）/ 8，其中Al为结构式M48 T34 O 85（OH）62的每个化学式单元的原子数，M和T分别为八面体和四面体位置）和基质中叶蛇纹石的平均XAl = 0.203 ± 0.019，导致510-530°C的峰变质温度受到很好的约束。峰值压力不太精确地限制在37 ± 7 kbar。因此，天山蛇纹岩记录了超高压变质条件，代表了迄今为止发现的最深俯冲蛇纹岩。在水镁石+叶蛇纹石+橄榄石+镁橄榄石的稳定区域内发生了退变质作用，以钛软骨石为代价形成了钛斜齿石，表明等温减压作用。P-T轨迹与围岩的变质演化过程吻合较好，表明天山超高压单元是作为一个相干块体俯冲折返的。为了进一步研究超镁铁岩的变质作用路径，我们利用活塞柱实验研究了钛软骨石和钛斜辉石的稳定性场。在25-55 kbar和600-750°C下，在无F的自然系统中进行了总共11个实验。结合以往的实验和信息，从天然岩石，我们构建了一个成岩网格的稳定性钛软骨石和钛斜齿石在无F橄榄岩组合物。蛇纹岩中钛软骨石的形成需要约26千巴的最低压力，而在富钛系统中，它可以在相当低的压力下形成。一个关键的发现是，在超高压条件下，无F的Ti-软骨石或Ti-斜腐植石在700至750°C之间在斜方辉石的存在下分解，温度显著低于这些腐植石矿物的最终分解反应的温度。这些分解反应是蛇纹岩俯冲过程中流体的额外来源。
The southwestern Tianshan (China) metamorphic belt records high-pressure (HP) to ultrahigh-pressure (UHP) conditions corresponding to a cold oceanic subduction-zone setting. Serpentinites enclosing retrogressed eclogite and rodingite occur as lenses within metapelites in the UHP unit, which also hosts coesite-bearing eclogites. Based on the petrology and petrography of these serpentinites, five events are recognized: (1) formation of a wehrlite–harzburgite–dunite association in the mantle; (2) retrograde metamorphism and partial hydration during exhumation of the mantle rocks close to the seafloor; (3) oceanic metamorphism leading to the first serpentinization and rodingitization; (4) UHP metamorphism during subduction; (5) retrograde metamorphism during exhumation together with a second serpentinization. The peak metamorphic mineral assemblage of the serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite + brucite. A computed pseudosection for this serpentinized wehrlite shows that the Al content in antigorite is mostly sensititive to temperature but can also be used to constrain pressure. The averageXAl= 0·204 ± 0·026 of antigorite (XAl= Al (a.p.f.u.)/8, where Al is in atoms per formula unit for a structural formula M48T34O85(OH)62, and M and T are octahedral and tetrahedral sites, respectively) included in Ti-chondrodite and averageXAl= 0·203 ± 0·019 of antigorite in the matrix result in a well-constrained peak metamorphic temperature of 510–530°C. Peak pressures are less precisely constrained at 37 ± 7 kbar. The Tianshan serpentinites thus record UHP metamorphic conditions and represent the deepest subducted serpentinites discovered so far. The retrograde evolution occurs within the stability field of brucite + antigorite + olivine + chlorite and formation of Ti-clinohumite at the expense of Ti-chondrodite has been observed, suggesting isothermal decompression. The resultingP–Tpath is in excellent agreement with the metamorphic evolution of country rocks, indicating that the UHP unit in Tianshan was subducted and exhumed as a coherent block. To refine the metamorphic path of the ultramafic rocks, we have investigated the stability fields of Ti-chondrodite and Ti-clinohumite using piston-cylinder experiments. A total of 11 experiments were conducted at 25–55 kbar and 600–750°C in a F-free natural system. Combined with previous experiments and information from natural rocks we constructed a petrogenetic grid for the stability of Ti-chondrodite and Ti-clinohumite in F-free peridotite compositions. The formation of Ti-chondrodite in serpentinites requires a minimum pressure of about 26 kbar, whereas in Ti-rich systems it can form at considerably lower pressures. A key finding is that at UHP conditions, F-free Ti-chondrodite or Ti-clinohumite breaks down in the presence of orthopyroxene between 700 and 750°C, at temperatures that are significantly lower than those of the terminal breakdown reactions of these humite minerals. These breakdown reactions are an additional source of fluid during prograde subduction of serpentinites.