Petrogenesis of basalts and gabbros from an ancient continent–ocean transition (External Liguride ophiolites, Northern Italy)
Petrogenesis of basalts and gabbros from an ancient continent–ocean transition (External Liguride ophiolites, Northern Italy)
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DOI：
10.1016/j.lithos.2007.09.007
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发表时间：
2008-03
期刊：
影响因子：
3.5
通讯作者：
A. Montanini;R. Tribuzio;L. Vernia
中科院分区：
文献类型：
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作者：
A. Montanini;R. Tribuzio;L. Vernia
与现代非火山大陆边缘相似的大陆-海洋过渡化石的残余物被保存在侏罗纪外利古里德单元中。它们由丰富的次大陆起源的伊尔热橄榄岩、莫尔型玄武岩和稀有的辉长岩侵入岩，以及在海洋化之前的裂谷期出土的大陆地壳体组成。辉长岩包括橄榄岩、辉长岩、含铁钛氧化物辉长岩和闪长岩。微量元素和Nd同位素组成表明，这些岩石是由N-MORB熔体通过分数结晶变化而形成的。在辉长岩中，沿韧性剪切带的高温（~ 900°C）剪切被角闪岩相重结晶（T ~ 650°C）局部覆盖，这很可能是由海水衍生的流体辅助的。玄武岩以熔岩流的形式出现，并以岩脉的形式横切地幔、辉长岩和辉长岩。它们显示出接近平坦的REE模式和较高的Y/Nb值（5-14），与现代N-MORB相似。玄武岩还具有相对于邻近的REE （Zr/Zr =1.1 ~ 1.7）和高（Sm/Yb） d比值（1.5 ~ 1.8）的弱Zr富集特征。它们的Nd同位素组成接近典型的枯竭地幔（初始Nd=+7.6 ~ +9.4）。玄武岩和辉长岩母熔体的地球化学特征可能归因于morb型软流圈源的熔融作用。微量元素模拟表明，贫尖晶石橄榄岩的低程度（≤6%）分数熔融不能解释玄武岩Sm/Yb比值升高的原因。提出了尖晶石橄榄岩与少量石榴石辉石岩混合源的低熔融现象来解释玄武岩的微量元素特征。
Remnants of a fossil continent–ocean transition similar to that of the modern non-volcanic continental margins are preserved in the Jurassic External Liguride units. They consist of fertile lherzolites of subcontinental origin, MOR-type basalts and rare gabbroic intrusives, together with continental crust bodies exhumed during the rifting phases preceding the oceanization. The gabbroic rocks include troctolites, (olivine) gabbros, Fe–Ti oxide-bearing gabbros and diorites. Trace element and Nd isotope compositions indicate that these rocks were derived from N-MORB melts variably evolved through fractional crystallisation. In the gabbroic rocks, high-temperature (∼900 °C) shearing along ductile shear zones is locally overprinted by amphibolite-facies recrystallization (T∼650 °C), which was most likely assisted by seawater-derived fluids. Basalts crop out as lava flows and as dykes crosscutting mantle lherzolites and gabbroic rocks. They display nearly flat REE patterns and high Y/Nb values (5–14), similar to modern N-MORB. Basalts are also characterised by weak Zr enrichment relative to neighbouring REE (Zr/Zr⁎=1.1–1.7) and high (Sm/Yb)DMratios (1.5–1.8). Their Nd isotope compositions are close to typical depleted mantle (initial ɛNd=+7.6 to+9.4). The geochemical features of parental melts of basaltic and gabbroic rocks may be attributed to melting of a MORB-type asthenospheric source. Trace element modelling shows that low-degree (≤6%) fractional melting of a depleted spinel peridotite cannot account for the elevated Sm/Yb ratios of basalts. Low-degree melting of a mixed source of spinel peridotite with small amounts of garnet pyroxenite has been proposed to explain the trace element signature of basalts.