Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons
Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons
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DOI：
10.1016/j.precamres.2019.105366
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发表时间：
2019-09
影响因子：
3.8
通讯作者：
E. Oliveira;C. Talavera;B. Windley;Lei Zhao;J. Semprich;N. McNaughton;W. Amaral;Gabriel Sombini;M. Navarro;Dailto Silva
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文献类型：
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作者：
E. Oliveira;C. Talavera;B. Windley;Lei Zhao;J. Semprich;N. McNaughton;W. Amaral;Gabriel Sombini;M. Navarro;Dailto Silva
区域规模的高压（HP）麻粒岩是由于构造事件而形成的，导致地壳增厚或地壳俯冲到地幔中。大多数HP麻粒岩是显生宙的，少数是元古代的，太古宙HP麻粒岩就更少了。在这里，我们展示了巴西圣弗朗西斯科克拉通 Uauá 地体的镁铁质麻粒岩和相关岩石的现场关系、矿物化学和锆石 U-Pb 年龄、Hf 同位素数据和微量元素数据，作为中太古代/新太古代过渡时期可能存在厚大陆壳的证据。 HP 镁铁质麻粒岩以透镜体形式出现在浅倾闪长岩到白闪长岩片麻岩中。镁铁质麻粒岩和闪长片麻岩之间的小规模层状表明这些岩石是同生的。石榴石-单斜辉石与石榴石中的石英、锆石、钛铁矿、斜长石和单斜辉石包裹体构成了 HP 组合的特征。石榴石成斑细胞还显示出 opx-cpx-plag 辛辉石冕，其与角闪石和斜长石相结合，定义了低品位麻粒岩和角闪岩相的 PT 条件。微探针数据与相平衡模型（伪剖面）相结合表明峰值 HP 组合为 16-18 kbar 和 930-960 C，低压麻粒岩到角闪岩相辛辉石冠为 6.2-7.0 kbar 和 660-760 C。与石榴石平衡的变质锆石边缘的SHRIMP年龄为2819±14 Ma，火成锆石核心的年龄为3127±14 Ma。伴生片麻岩样品中锆石岩心的 U-Pb 年龄在 3090±13 Ma (LA-ICP-MS) 和 3125±15 Ma (SHRIMP) 之间，年龄簇为 3120±6 Ma。高压镁铁质麻粒岩火成锆石的 εHf (t) 值略为正，而变质锆石边缘为负。伴生的闪长岩片麻岩总是产生负的锆石 εHf (t) 值。侵入镁铁质麻粒岩中的特隆兹铁矿片大约为 100 片。比宿主麻粒岩年轻20（2794±13 Ma）。我们将镁铁质麻粒岩和白闪长片麻岩的火成岩原岩解释为位于大陆地壳中的单一火成岩杂岩，后来在块体/克拉通碰撞期间因收缩和地壳增厚而变形和变质，在中太古代末期形成地球上第一个超级克拉通。
High pressure (HP) granulites of regional scale form as a result of tectonic events that lead to crustal thickening or subduction of the crust into the mantle. Most HP granulites are Phanerozoic, a few are Proterozoic, and Archaean HP granulites are even scarcer. Here we present field relationships, mineral chemistry and zircon U-Pb ages, Hf isotope data and trace elements data for the mafic granulite and associated rocks of the Uauá terrane, São Francisco Craton, Brazil, as evidence for the likely existence of a thick continental crust in the Mesoarchaean/Neoarchaean transition. The HP mafic granulite occurs as lensoid bodies within shallow dipping diorite to leucodiorite gneisses. Small scale layering between mafic granulite and diorite gneiss indicate these rocks are cogenetic. Garnet-clinopyroxene pairs with quartz, zircon, ilmenite, plagioclase, and clinopyroxene inclusions in garnet characterize the HP assemblage. Garnet porphyroblasts also show opx-cpx-plag symplectite coronas, which coupled with hornblende and plagioclase define PT conditions to lower grade granulite and amphibolite facies. Microprobe data combined with phase equilibria modelling (pseudosections) indicate 16–18 kbar and 930–960 C for the peak HP assemblage, and 6.2–7.0 kbar and 660–760 C for lower pressure granulite to amphibolite facies symplectite coronas. Metamorphic zircon rims in equilibrium with garnet have SHRIMP ages of 2819±14, and igneous zircon cores of 3127±14 Ma. The cores of zircon in associated gneiss samples have U-Pb ages between 3090±13 Ma (LA-ICP-MS) and 3125±15 Ma (SHRIMP) with age cluster at 3120±6 Ma. εHf (t) values on igneous zircon of the HP mafic granulite are slightly positive whereas metamorphic zircon rims are negative. The associated diorite gneiss invariably yielded negative zircon εHf (t) values. Trondhjemite sheets intrusive in the mafic granulite are ca. 20 my younger (2794±13 Ma) than the host granulite. We interpret the igneous protoliths of the mafic granulite and leucodiorite gneiss as a single igneous complex emplaced in the continental crust, later deformed and metamorphosed by contraction and crustal thickening during collision of blocks/cratons to form Earth's first supercratons by the end of the Mesoarchaean.