Mafic dykes intrusive into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt (Parnaíba Province), north–northeastern Brazil: Geochemistry, Sr–Nd–Pb–O isotopes, 40Ar/39Ar geochronology, and relationships to CAMP magmatism
Mafic dykes intrusive into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt (Parnaíba Province), north–northeastern Brazil: Geochemistry, Sr–Nd–Pb–O isotopes, 40Ar/39Ar geochronology, and relationships to CAMP magmatism
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侵入巴西北部至东北部圣路易斯克拉通碎片和古鲁皮带（帕尔纳伊巴省）前寒武纪岩石的镁铁质岩脉：地球化学、Sr–Nd–Pb–O 同位素、40Ar/39Ar 地质年代学以及与 CAMP 岩浆作用的关系
DOI：
10.1016/j.lithos.2013.04.015
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发表时间：
2013
期刊：
影响因子：
3.5
通讯作者：
M. Babinski
中科院分区：
文献类型：
--
作者：
E. Klein;R. Angélica;C. Harris;F. Jourdan;M. Babinski
辉绿岩和微辉长岩岩脉侵入圣路易斯克拉通碎片和古鲁皮带的前寒武纪岩石，这些岩石是巴西东北部帕纳伊巴盆地的构造和侵蚀窗口。测定了 Ar-Ar 年龄，并测量了这些岩脉的主量元素、微量元素和 Nd-Sr-Pb-O 同位素组成，以深入了解它们的年龄、地幔来源的性质和成岩过程。这些数据还用于将岩脉的化学和同位素特征与中大西洋岩浆省（CAMP）的化学和同位素特征进行比较。已经确定了镁铁质岩脉的四个化学组。这些岩石包括两种高钛岩石亚型 (i) HTi-1 (TiO2<2.3wt.%; SiO2>47 wt.%)，(ii) HTi-2 (TiO2>2.7wt.%; SiO2>47wt.%)，此外还有 (iii) 演化出的高钛 (TiO2>4wt.%; SiO2 ~46wt.%) 和 (iv) 低硅（TiO2>2.2wt.%；SiO2<45wt.%）岩石。斜长石的40Ar/39Ar年代学返回HTi-2亚型的年龄为201±4 Ma和193±10 Ma，以及演化的高Ti组的201±2 Ma和207±9Ma。 HTi-1 和低硅组呈现出高度干扰的年龄谱，并且不允许定义它们的侵位年龄。氩数据表明低硅组的年龄>200Ma，并且对于HTi-1亚型的年龄是可疑的，如果与HTi-2和演化的高Ti类型同时存在（即~200Ma）或更早。所有组都呈现出与未受污染的幔源岩浆相容的辉石 δ18O 值。 HTi-1亚型（平均143Nd/144Nd200=0.512644；87Sr/86Sr200=0.7035；206Pb/204Pb为17.86）显示所有组中富集程度较低且分馏程度较低（更原始）的微量元素分布。 HTi-2 亚型显示出富集的微量元素模式和贫化的 Nd-Sr 特征（143Nd/144Nd200=0.512610；87Sr/86Sr200=0.7037），平均 206Pb/204Pb 比率为 17.23。演化出的高Ti化学族的平均比值为143Nd/144Nd200=0.512558、87Sr/86Sr200=0.7035、206Pb/204Pb为16.88，四族中微量元素特征更加丰富。 HTi-1/HTi-2/EHTi 类型的化学和同位素组成及趋势与它们源自软流圈的母岩浆的衍生一致，该母岩浆通过分异和最小地壳污染（HTi-2 和 EHTi 类型较高）进一步改变，并且通过分步结晶从一种类型衍生出另一种类型。这些高 TiO2 类型和演化的高 TiO2 类型显示出与 CAMP 岩浆作用一致的年龄以及一些化学和同位素特征。发现的一些差异归因于岩石成因过程，例如岩浆分化。埃迪卡拉纪-寒武纪边界的新元古代（巴西/泛非）造山运动闭合后，地幔变暖与盘古大陆超大陆下方边缘驱动的对流相结合，可能引发了岩浆事件。低硅型显示出成对的 Ta-Nb 和 Zr-Hf 贫化，以及贫化 Sr-Nd（平均 143Nd/144Nd200=0.512687；87Sr/86Sr200=0.703）和富集 Pb（206Pb/204Pb 为 18.66）同位素组成，这些同位素组成可解释为由次大陆岩石圈地幔，具有早期俯冲的产物或在大陆地壳上升和就位期间受到地幔衍生岩浆污染的产物。据推测，这些岩脉位于埃迪卡拉纪-寒武纪边界，在形成古鲁皮带的新元古代造山运动之后，以及在帕尔纳伊巴盆地形成之前的早期伸展阶段。
Dykes of diabase and microgabbro intruded into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt, which are tectonic and erosive windows of the Parnaíba Basin in north–northeastern Brazil. Ar–Ar ages were determined, and major, trace element, and Nd–Sr–Pb–O isotopic compositions of these dykes were measured to provide insights into their age, and into the nature of their mantle sources and petrogenetic processes. The data have also been used to compare the chemical and isotopic signatures of the dykes with those of the Central Atlantic Magmatic Province (CAMP). Four chemical groups of mafic dykes have been identified. These comprise two subtypes of high-Ti rocks (i) HTi-1 (TiO2<2.3wt.%; SiO2>47 wt.%), (ii) HTi-2 (TiO2>2.7wt.%; SiO2>47wt.%), in addition to (iii) evolved high-Ti (TiO2>4wt.%; SiO2of ~46wt.%) and (iv) low-Si (TiO2>2.2wt.%; SiO2<45wt.%) rocks.40Ar/39Ar geochronology of plagioclase returned ages of 201±4 Ma and 193±10 Ma for the HTi-2 subtype, and of 201±2 Ma and 207±9Ma for the evolved high-Ti group. The HTi-1 and low-Si groups presented highly disturbed age spectra, and did not allow the definition of their emplacement ages. The Argon data indicate an age >200Ma for the low-Si group and are dubious with respect to the age of theHTi-1 subtype, if coeval with (i.e., ~200Ma), or older than, the HTi-2 and evolved high-Ti types. All groups present δ18O values of pyroxene that are compatible with uncontaminated mantle-derived magmas. The HTi-1 subtype (average143Nd/144Nd200=0.512644;87Sr/86Sr200=0.7035;206Pb/204Pb of 17.86) shows the less enriched and less fractionated (more primitive) trace element distribution of all groups. The HTi-2 subtype shows enriched trace element pattern and depleted Nd–Sr signature (143Nd/144Nd200=0.512610;87Sr/86Sr200=0.7037) and average206Pb/204Pb ratios of 17.23. The evolved high-Ti chemical group shows average ratios of143Nd/144Nd200=0.512558,87Sr/86Sr200=0.7035, and206Pb/204Pb of 16.88, and the more enriched trace-elements signature among the four groups. The chemical and isotopic compositions and trends of the HTi-1/HTi-2/EHTi types are consistent with their derivation from an asthenosphere-derived parental magma further modified by differentiation and minimal crust contamination (higher in the HTi-2 and EHTi types), and by the derivation of one type from another via fractional crystallization. These high- and evolved high-TiO2types show ages and some chemical and isotopic features that are consistent with those of the CAMP magmatism. Some differences found are ascribed to petrogenetic processes, such as magma differentiation. A combination of warming of the mantle and edge-driven convection beneath the Pangea supercontinent after the closure of the Neoproterozoic (Brasiliano/Pan-African) orogenies in the Ediacaran–Cambrian boundary might have triggered the magmatic event. The low-Si type shows paired Ta–Nb and Zr–Hf depletions, and depleted Sr–Nd (average143Nd/144Nd200=0.512687;87Sr/86Sr200=0.703) and enriched Pb (206Pb/204Pb of 18.66) isotopic compositions that may be interpreted to result either from interaction of a subcontinental lithospheric mantle with products of an earlier subduction or by contamination of the mantle-derived magma during ascent and emplacement in the continental crust. It is hypothesized that these dykes were emplaced in the Ediacaran–Cambrian boundary, after the Neoproterozoic orogeny that built up the Gurupi Belt and in the early extensional stages that preceded the formation of the Parnaíba Basin.