Conditions Of Formation Of Charnockitic Magmatic Rocks From The Várzea Alegre Massif, Espírito Santo, Southeastern Brazil
Conditions Of Formation Of Charnockitic Magmatic Rocks From The Várzea Alegre Massif, Espírito Santo, Southeastern Brazil
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巴西东南部圣埃斯皮里图州瓦尔泽亚阿莱格里地块的霞诺岩岩浆岩的形成条件
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发表时间：
1999
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通讯作者：
I. McReath
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作者：
J. Mendes;C. M. Wiedemann;I. McReath
位于圣埃斯皮里托州中部的瓦尔泽亚阿莱格雷侵入岩与该地区晚期至构造后的巴西利亚诺岩浆弧有关。它是一个几乎呈圆形的反环带岩体，被变质为角闪岩和麻粒岩相的正、副片麻岩包围。岩体有一个辉长岩中心，周围有闪长岩/石英闪长岩、二长闪长岩和巨斑花岗岩。所有这些岩浆岩都被一个大而不规则的碳硅石岩石环（英安岩、粗面玄岩、石英闪长岩和石英镁橄榄石）所包围。紫苏辉石岩由斜长石、条纹长石/中条纹长石、斜方辉石、黑云母、角闪石、钛铁矿、磁铁矿、黄铁矿、磷灰石、锆石、稀有褐帘石和赤铁矿组成。这些岩石的化学成分指向高钾钙碱性岩浆富集LIL和HFS元素。斜方辉石（Wo 1.6 -2.5，En 30，4-41.2至Fs 57 -67.4）部分被黑云母、角闪石和不透明矿物取代。角闪岩（镁铁角闪石到镁铁角闪石）的铝含量变化有限。黑云母的成分接近铁云母端元，具有高Ti含量。斜方辉石、角闪石和黑云母的X_（IG大于寄主全岩的X_（ig）值。巨晶长石和来自基质的巨晶呈现相似的成分变化。斜长石（Ab 65 An 32至Ab 57 An 40）在Or分子中显示出小的变化。碱性长石（（Or 89 Ab 11至Or 69 Ab 31）的成分变化与Na组分的不平等出溶有关。瓦尔泽亚阿莱格雷碳硅石的Zr和P2 O 5饱和度表明岩浆结晶温度约为950°C。通过钛铁矿-磁铁矿和斜长石-碱性长石对估计的亚固相线温度在550°C和630°C之间。估计的结晶压力范围为6.5至7千巴。计算的fO 2是一致的高度还原条件下，这是由岩石的低Mg/（Mg+Fe）比证实。
The Varzea Alegre intrusive massif, in the central portion of the Espirito Santo State, is related to the late to post-tectonic Brasiliano magmatic arc in this region. It is an inversely zoned pluton with an almost circular shape, enclosed by ortho and paragneisses metamorphosed to the amphibolite and granulite facies. The pluton has a gabbroic center surrounded by diorites/quartz diorites, monzodiorites and megaporphyritic granites. All these magmatites are involved by a large and irregular ring of charnockitic rocks (opdalites, jotunites, opx-quartz diorites and quartz mangerites). The charnockites are composed of plagioclase, perthitic alkali feldspar/mesoperthite, orthopyroxene, biotite, hornblende, ilmenite, magnetite, pyrite, apatite, zircon, rare allanite and hematite. Chemical composition of these rocks points towards a high-K calc-alkalic magmatism enriched in LIL and HFS elements. Orthopyroxene (Wo1.6-2.5, En30,4-41.2 to Fs57-67.4) is partly replaced by biotite, amphibole and opaque minerals. Amphiboles (Mg-hastingsitic hornblende to magnesian hastingsite) show limited variations in the Al-contents. Biotite has compositions close to the annite end member, with high Ti-contents. The X\ig of orthopyroxene, amphibole and biotite are larger than those calculated for host whole rocks. Megacrysts feldspar and from the matrix present similar compositional variation. Plagioclase (Ab65An32 to Ab57An40) show small variations in the Or molecule. Compositional variations in the alkali feldspar ((Or89Ab11 to Or69Ab31) are associated to an unequal exsolution of the Na component. Zr and P2O5 saturation levels of the Varzea Alegre charnockites indicate magmatic crystallization temperature around 950°C. Subsolidus temperatures, estimated through the ilmenite-magnetite and plagioclase-alkali feldspar pairs, are between 550°C and 630°C. Estimated crystallization pressure ranges from 6.5 to 7 kbar. Calculated fO2 is consistent with highly reducing conditions, which is confirmed by the low Mg/(Mg+Fe) ratios of the rocks.