The origin of low δ18O granites and related rocks from the Seychelles
The origin of low δ18O granites and related rocks from the Seychelles
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DOI：
10.1007/s00410-002-0349-6
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发表时间：
2002-06-01
影响因子：
3.5
通讯作者：
Ashwal, LD
中科院分区：
文献类型：
--
作者：
Harris, C;Ashwal, LD
750-Ma塞舌尔花岗岩的全岩三角洲（18）O值范围为-1.2至+7.5ppm。石英、长石和全岩δ（18）O值在6180范围内的差异表明，这些矿物处于岩浆平衡状态，全岩δ（18）O值受结晶后相互作用和流体的影响很小。两套花岗岩（马埃岛型和Praslin型）已被确认的化学和放射性同位素组成的基础上。前者的平均全岩δ（18）O值为5.25 +/-0.65parts per thousand（1sigma），而后者的变化更大，平均全岩PO值为3.00 +/-3.27parts per thousand。两种花岗岩中黑云母和角闪石的δ D值在-69~-116ppm之间，与全岩δ（18）O值呈正相关。在结晶过程中或结晶后不久侵入花岗岩的白云岩脉的全岩PO值（平均为2.24 +/-1.93ppm）也低于幔源玄武岩的预期值，但这些值似乎是流体-岩石相互作用的结果。我们认为，Mahe型花岗岩主要来源于具有"正常" δ D和δ（18）O值的年轻镁铁质到中地壳，而Praslin型花岗岩则是这种源岩与较老地壳的混合物，后者通过与大气降水在高温下的广泛相互作用获得了较低的δ D和δ（18）O值。由于当时该地区的纬度相对较低，因此在750 Ma时不太可能获得D和O-18中充分耗尽的大气降水。我们认为源区的蚀变发生在花岗岩岩浆产生之前。在O-18塞舌尔花岗岩的消耗并不一定需要在750马的区域伸展构造环境，已经提出。
The 750-Ma Seychelles granites have whole-rock delta(18)O values that range from -1.2 to + 7.5parts per thousand. Differences in delta(18)O values between quartz, feldspar and whole-rock for samples that have a range of 6180 values suggest that these minerals are in magmatic equilibrium, and that the whole-rock delta(18)O values were little affected by post-crystallization interaction,kith fluids. Two suites of granites (the Mahe type and the Praslin type) have previously been recognized on the basis of chemical and radiogenic isotope composition. The former have a mean whole-rock delta(18)O value of 5.25 +/- 0.65parts per thousand (1sigma), whereas the latter are much more variable, with a mean whole-rock PO value of 3.00 +/- 3.27parts per thousand. Biotite and amphibole separates from both granite types range in deltaD from -69 to -116parts per thousand, and show a positive correlation with whole-rock delta(18)O values. Dolerite dykes that intruded the granites during or shortly after their crystallization also have whole-rock PO values (mean 2.24 +/- 1.93parts per thousand) lower than that expected for mantle-derived basaltic rocks, but these values appear to be the result of fluid-rock interaction. We suggest that the Mahe-type granites are derived mainly from juvenile mafic to intermediate crust with 'normal' deltaD and delta(18)O values, whereas the Praslin-type granites are mixtures of this source and older crust that acquired its low deltaD and delta(18)O values by extensive interaction with meteoric water at high temperature. It is unlikely that meteoric water sufficiently depleted in D and O-18 was available at 750 Ma because of the relatively low latitude of the region at that time. We suggest that alteration of the source took place significantly before production of the granite magmas. Depletion in O-18 of the Seychelles granites does not necessarily require a regional extensional tectonic setting at 750 Ma, as has been proposed.