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首页> 外文期刊>Journal of South American earth sciences >Trace element composition of parental magmas from mafic-ultramafic cumulates determined by in situ mineral analyses: The Juquia mafic-ultramafic alkaline-carbonatite massif, SE Brazil
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Trace element composition of parental magmas from mafic-ultramafic cumulates determined by in situ mineral analyses: The Juquia mafic-ultramafic alkaline-carbonatite massif, SE Brazil

机译：通过原位矿物分析确定的镁铁矿-超镁铁矿堆积物的父母岩浆的微量元素组成：巴西东南部的Juquia镁铁矿-超镁铁矿碱性碳酸盐岩地块

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摘要

Major and trace element analyses of relict cores of cumulus minerals (olivine and dinopyroxene) from primitive rocks of the Juquia mafic-ultramafic alkaline-carbonatite complex (Early Cretaceous) in the Ponta Grossa Arch Alkaline Province, southeastern Brazil, were used to calculate the equilibrium melt compositions. Olivine relict cores are compositionally restricted to the Fo_(83-74) interval, and they exhibit significant concentrations of Mn (2220-3001 ppm), Ni (1188-2327 ppm), Ca (175-649 ppm), Co (169 -216 ppm), Zn (115-215 ppm), Ti (55-305 ppm), Cr (4-320 ppm) and P (31-154 ppm). Clinopyrox-ene relict cores are essentially diopside, with minor amounts of hedenbergite and tschermakite components. The trace element contents found in dinopyroxene cores are Ni (196-339 ppm), V (99 -318 ppm), Sr (110-260 ppm), Zr (12-163 ppm), Sc (50-78 ppm), P (16-118 ppm), Zn (16-48 ppm) and Co (25-43 ppm). The rare earth element (REE) concentrations are variable (28-240 ppm), with the lighter elements present in the greatest abundance, particularly Ce (up to 83 ppm). The melt compositions in equilibrium with olivine and dinopyroxene cores from different samples suggest that distinct basanite magma batches have played a role in the formation of the Juquia cumulate rocks. These calculated liquids have compositions that are quite similar to those of the nearby lamprophyre dikes. The CaO/Al_2O_3 values (>0.8) from the lamprophyre dikes and the high La/Zr and low Ti/Eu values from the calculated liquids point to a previous carbonatite metasomatic event in the magmatic source that lead to the formation of wehrlite veins in a peridotitic mantle. The variation in the Hf/Y ratio implies differences in cpx/gt modal proportions in the source. Non-modal mantle batch-melting models indicate that a homogeneous source with little variation in the degree of partial melting cannot explain the trace element differences among the calculated liquids. The models suggest that the geochemical differences represent (1) mixing between metasomatic vein partial melt and garnet lherzolite partial melt and (2) clinopyroxene/garnet modal ratios of the residual mantle.
机译：巴西东南部蓬塔格罗萨弓碱省的Juquia镁铁质-超镁铁碱性碳酸盐复合体（早白垩世）原始岩石中的积云矿物（橄榄石和二茂铁）的残留岩心的主要元素和微量元素分析用于计算平衡熔体成分。橄榄石遗迹岩心的成分限制在Fo_（83-74）范围内，并且它们显示出显着浓度的Mn（2220-3001 ppm），Ni（1188-2327 ppm），Ca（175-649 ppm），Co（169- 216 ppm），Zn（115-215 ppm），Ti（55-305 ppm），Cr（4-320 ppm）和P（31-154 ppm）。 Clinopyrox-ene的遗迹岩心基本上是透辉石，含有少量的水辉石和钙锰矿。在二茂铁芯中发现的痕量元素含量为Ni（196-339 ppm），V（99 -318 ppm），Sr（110-260 ppm），Zr（12-163 ppm），Sc（50-78 ppm），P （16-118 ppm），Zn（16-48 ppm）和Co（25-43 ppm）。稀土元素（REE）的浓度是可变的（28-240 ppm），其中较轻的元素以最大的丰度存在，尤其是Ce（最高83 ppm）。熔体组成与来自不同样品的橄榄石和二茂铁芯处于平衡状态，这表明不同的玄武岩岩浆批次在Juquia堆积岩的形成中发挥了作用。这些计算出的液体的成分与附近的煌斑岩堤防非常相似。煌斑岩堤的CaO / Al_2O_3值（> 0.8）和计算液体的高La / Zr和低Ti / Eu值指向岩浆源中先前的碳酸盐岩交代事件，导致在岩浆中形成白蜡岩脉。橄榄岩幔。 Hf / Y比的变化意味着源中cpx / gt模态比例的差异。非模态地幔分批熔融模型表明，部分熔融程度几乎没有变化的均质源无法解释计算出的液体之间的痕量元素差异。这些模型表明，地球化学差异代表（1）交代静脉部分熔融与石榴石锂铁矿部分熔融之间的混合以及（2）残留地幔的斜辉石/石榴石模态比。

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