Composition and SmNd isotopic data of the lower crust beneath San Luis Potosí, central Mexico: Evidence from a granulite-facies xenolith suite
Composition and SmNd isotopic data of the lower crust beneath San Luis Potosí, central Mexico: Evidence from a granulite-facies xenolith suite
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墨西哥中部圣路易斯波托西下地壳的成分和 SmNd 同位素数据：来自麻粒岩相捕虏体套件的证据
DOI：
10.1016/0009-2541(94)90170-8
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发表时间：
1994
期刊：
影响因子：
3.9
通讯作者：
T. Besch
中科院分区：
文献类型：
--
作者：
P. Schaaf;W. Heinrich;T. Besch
墨西哥中部圣路易斯波托西（SLP）的第四纪火山岩采样了一套捕虏体，包括上永久结核（尖晶石二辉石岩、角闪石辉石岩、尖晶石辉石岩）和下地壳捕虏体。后者由麻粒岩相、镁铁质至中等变质火成岩和高级变质沉积物组成。变质岩系的前身从辉长岩到英云闪长岩。全岩REE模式可将变火成岩分为堆积岩和分馏熔体两类，镁铁质捕虏体由斜长石±单斜辉石±斜方辉石±石榴石±角闪石±磷灰石±金红石±方柱石组成。石榴石与单斜辉石发生了广泛的反应，形成了次生斜长石、斜方辉石和角闪石的冕状结构。中间包体不含石榴石，含石英。所有样品都显示出在斜方辉石、单斜辉石和斜长石中记录的相似的Ca-Al分带模式，这些模式沿着石榴石周围的日冕的存在，被解释为代表从940 ± 60°C和7-11 kbar到较低温度和压力的部分再平衡阶段。尖晶石二辉橄榄岩的Sm-Nd同位素组成表明SLP之下地幔段的成分不均匀。类似的Sr-和Nd-同位素特征表明，角闪石辉石岩的高压堆与主机玄武岩。在Sm-Nd等时线图上，四个不含次生角闪石的中间变质麻粒岩和两个被认为是变质岩系最原始成员的尖晶石辉石岩定义了一条回归线，其年龄值为1248 ± 69 Ma，含次生角闪石的变质岩包体的初始Nd值为+2.3，明显高于回归线。这可能是由于在退变质条件下角闪石形成过程中引入了外源Nd。讨论了计算的年龄值的地质意义，有一些论点解释这个年龄代表岩浆前兆的侵入年龄。这提供了证据，形成下地壳的基本到中间组成在中元古代，现在位于SLP之下。
Quaternary volcanics from San Luis Potosí (SLP) in central Mexico sampled a xenolith suite comprising uppermantle nodules (spinell lherzolites, hornblende pyroxenites, spinel pyroxenites) and lower-crustal xenoliths. The latter consist of granulite-facies, mafic to intermediate metaigneous rocks and high-grade metasediments. Precursors of the metaigneous suite range from gabbroic to tonalitic compositions. Whole-rock REE patterns permit classification of metaigneous rocks into cumulates and fractionated melts.Mafic xenoliths consist of plagioclase ± clinopyroxene ± orthopyroxene ± garnet ± amphibole ± apatite ± rutile ± scapolite. Garnet has reacted extensively with clinopyroxene, forming corona textures of secondary plagioclase, orthopyroxene and amphibole. Intermediate xenoliths are garnet-free and quartz-bearing. All samples display similar Ca-Al zoning patterns documented in orthopyroxene, clinopyroxene and plagioclase which are, along with the presence of coronas around garnet, interpreted as representing partial re-equilibration stages from 940 ± 60°C and 7–11 kbar down to lower temperatures and pressures. There is no indication of a secondary heating event in these xenoliths.Sm-Nd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath SLP. Similar Sr- and Nd-isotopic signatures reveal that the hornblende pyroxenites are high-pressure cumulates linked to the host basanites. Four mainly intermediate metaigneous granulites from a single vent, which are free of secondary amphibole, and two spinel pyroxenites, regarded as the most primitive member of the metaigneous suite, define a regression line in a Sm-Nd isochron diagram, yielding an age value of 1248 ± 69 Ma and an initialϵNd-value of +2.3 Metaigneous xenoliths containing secondary amphibole plot significantly above the regression line, which is probably due to the introduction of externally derived Nd during amphibole formation under retrogradep-Tconditions. Discussing the geological significance of the calculated age value, there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors. This gives evidence for formation of lower crust of basic to intermediate compositions during the mid-Proterozoic, now situated beneath SLP.