Importance of late-magmatic and hydrothermal fluids on the Sm–Nd isotope mineral systematics of hypersolvus granites
Importance of late-magmatic and hydrothermal fluids on the Sm–Nd isotope mineral systematics of hypersolvus granites
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DOI：
10.1016/s0009-2541(98)00010-2
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发表时间：
1998-05
期刊：
影响因子：
3.9
通讯作者：
F. Poitrasson;J. Paquette;J. Montel;C. Pin;J. Duthou
中科院分区：
文献类型：
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作者：
F. Poitrasson;J. Paquette;J. Montel;C. Pin;J. Duthou
尽管 Sm-Nd 辐射测量系统在同位素地球化学的各个领域产生了重大影响，但很少有人尝试在花岗岩研究的矿物尺度上使用它。这种方法已应用于来自科西嘉岛（法国东南部）非造山省的两块花岗岩。在 Mantelluccio 含超溶铁橄榄石花岗岩的等时线图中获得了两个统计上可接受的排列。碱长石、角闪石、无夹杂物锆石、萤石和镁橄榄石的年龄为 331±10 Ma，而钠长石、全岩、角闪石和无夹杂物锆石的年龄则明显较年轻，为 291±13 Ma。后者通过 283±1 Ma 的锆石 U-Pb 年龄作为花岗岩的岩浆侵位日期得到验证。另一种排列给出了 50 Ma 的年龄，可以解释为继承的，或者更可能的是，在花岗岩结晶结束时热液对流与围岩相互作用的结果。绿帘石的 Sm-Nd 系统学记录了第二次流体-岩石事件，该事件可能发生在花岗岩侵位后至少 200 Ma。极低的 143Nd/144Nd 比率表明绿帘石是从在地壳中携带镧系元素至少 1 公里距离的流体中结晶而成的。含有夹杂物的锆石比不含夹杂物的锆石具有更低的初始 143Nd/144Nd 比率。这一特征归因于地壳中花岗岩熔体上升过程中通过污染而继承的磷灰石和/或锆石核心。 Evisa 的超溶线过碱性花岗岩也获得了两条等时线。角闪石、萤石、榍石和辉格闪石的年龄为 259±6 Ma，而全岩、碱长石和变晶锆石的年龄为 209±14 Ma。在这种情况下，前一个值与可能稍微更新的全岩 Rb-Sr 日期 249±3 Ma 大致一致，表明它记录了花岗岩的岩浆结晶年龄。碱长石、变晶锆石和可能的铜长石的 Sm-Nd 同位素系统在侏罗纪早期影响西欧的一次重要热液事件中重新平衡。然而，在这两个例子中，全岩 Nd 同位素特征似乎并未受到各种流体-岩石事件的强烈影响，这表明它们仍然可以用于岩石成因研究。相比之下，这些结果表明，像 K-Ar 和 Rb-Sr 一样，对岩浆后过程抵抗力较弱的矿物的 Sm-Nd 系统可用于追踪和测定热液事件。
Despite the significant impact of the Sm–Nd radiometric system in various fields of isotope geochemistry, very few attempts have been made to use it at the mineral scale in granite studies. This approach has been used on two granites from the anorogenic province of Corsica (SE France). Two statistically acceptable alignments were obtained in an isochron diagram for the hypersolvus fayalite-bearing granite of Mantelluccio. Alkali–feldspar, amphibole, inclusion-free zircon, fluorite and fergusonite give an age of 331±10 Ma, whereas allanite, whole-rock, amphibole and inclusion-free zircon yield a significantly younger age at 291±13 Ma. The latter is validated by a zircon U–Pb age of 283±1 Ma as the magmatic emplacement date of the granite. The other alignment, giving a 50-Ma older age, can be interpreted either as being inherited or, more likely, as the result of hydrothermal convection interacting with the country rocks at the end of granite crystallization. The Sm–Nd systematics of epidote records a second fluid–rock event, which probably occurred at least 200 Ma after the emplacement of the granite. Extremely low143Nd/144Nd ratio indicates that the epidote crystallized from a fluid that carried lanthanides in the crust over a distance of at least 1 km. Zircons with inclusions have a lower initial143Nd/144Nd ratio than inclusion-free zircons. This feature is attributed to apatite and/or zircon core inheritance, through contamination, during the ascent of the granitic melt in the crust. Two isochrons have been also obtained for the hypersolvus peralkaline granite of Evisa. Amphibole, fluorite, sphene and fergusonite yield an age of 259±6 Ma, whereas the whole-rock, alkali–feldspar and metamict zircon give an age at 209±14 Ma. In this case, the former value is roughly in agreement with a possibly slightly rejuvenated whole-rock Rb–Sr date of 249±3 Ma, suggesting that it documents the magmatic crystallization age of the granite. Sm–Nd isotope systematics of the alkali–feldspar, metamict zircon and possibly allanite, were re-equilibrated during an important hydrothermal event that affected Western Europe in Early Jurassic times. In both examples, however, it appears that the whole-rock Nd isotopic signatures were not strongly affected by the various fluid–rock events, indicating that they can still be used for petrogenetic investigations. In contrast, these results show that, like K–Ar and Rb–Sr, the Sm–Nd systematics of minerals with weak resistance to post-magmatic processes can be used to trace and date hydrothermal events.