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首页> 外文期刊>Lithos: An International Journal of Mineralogy, Petrology, and Geochemistry >Igneous banding, schlieren and mafic enclaves in calc-alkaline granites: The Budduso pluton (Sardinia)
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Igneous banding, schlieren and mafic enclaves in calc-alkaline granites: The Budduso pluton (Sardinia)

机译：钙碱性花岗岩中的火成岩带，裂纹岩和镁铁质飞地：Budduso岩体（撒丁岛）

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摘要

This study deals with the origin of igneous layering in plutons, and, especially, the extent layering is related to mafic-silicic magma interactions. The Budduso pluton (Sardinia) shows three main scales of organization. (i) Large scale lithological variations correspond to three main magmatic units, with differentiation increasing from the Outer (hornblende-bearing biotite granodiorite/monzogranite) to the Middle (biotite monzogranite) and the Inner (leucomonzogranite) units. The striking homogeneity of ~(87)Sr/~(86)Sr initial ratios (0.7090±4) and ε_(Nd(t)) values (—5.6±0.1) strongly suggests that magma isotopic equilibration was achieved prior to emplacement, whereas mixing/mingling structures observed within the pluton reflect second-stage processes involving broadly oogenetic components. (ii) Metre to decametre-scale igneous layering may be isomodal or modally-graded, locally with cross-layering. Biotite and plagioclase compositions are similar in both biotite-rich and quartzofeldspathic layers, as are the trace-element patterns which differ only by relative abundances. This precludes an origin by fractional crystallization. A penetrative submagmatic fabric superimposed on the layering and corresponding mainly to flattening can be ascribed to interference between pluton growth and regional deformation. (iii) Composite layering and schlieren are commonly associated to mafic microgranular enclaves, locally within synmagmatic shear zones or disrupted synplutonic dykes. In that case, there is a progressive shift in biotite X_(Fe) values from the core of enclave (～0.65) to the host monzogranite (～ 0.72): schlieren in the monzogranite show biotite X_(Fe) values similar to that of the host rock, whereas schlieren close to mafic enclaves show lower X_(Fe) values (～ 0.69) towards those of enclave rims. These features can be ascribed to three main processes: (i) assembly of differentiated (± mixed/mingled) magmatic pulses; (ii) local hydrodynamic sorting related to density currents in a mush, and segregation of residual me (iii) mechanical disruption and chemical hybridization of mafic magmas during ascent or within the pluton related to magma dynamics. None of these processes affect the whole pluton but they are limited to specific magmatic units. Therefore, pluton growth by incremental assembly of magma batches is not incompatible with magma chamber processes.
机译：这项研究处理了火成岩中火成岩分层的起源，特别是分层与镁铁质-硅质岩浆相互作用有关。 Budduso pluton（撒丁岛）显示了三个主要的组织规模。 （i）大型岩性变化对应于三个主要的岩浆单元，从外（含角闪石的黑云母花岗闪长岩/辉长花岗岩）向中（黑云母的辉长花岗岩）和内（白云母花岗岩）单元的分化程度增加。 〜（87）Sr /〜（86）Sr初始比率（0.7090±4）和ε_（Nd（t））值（-5.6±0.1）的惊人同质性强烈表明，岩浆同位素平衡是在沉积前实现的，而在子体中观察到的混合/混合结构反映了涉及广泛成岩成分的第二阶段过程。 （ii）米至百米尺度的火成岩层可以是等峰的或模态渐变的，局部带有交叉层。在富含黑云母的层和石英长石层中，黑云母和斜长石的组成都相似，痕量元素模式也仅因相对丰度而不同。这排除了通过分步结晶的起源。叠加在分层上且主要对应于扁平化的穿透性亚岩浆织物可归因于胶体生长与区域变形之间的干扰。 （iii）复合层和白垩纪通常与铁镁质微颗粒飞地有关，局部位于岩浆剪切带或断裂的岩浆岩堤内。在这种情况下，黑云母X_（Fe）值从飞地的中心（〜0.65）逐渐过渡到主体蒙脱花岗岩（〜0.72）：蒙脱石中的schlieren显示黑云母X_（Fe）的值类似于基质岩，而接近镁铁质飞地的schlieren的X_（Fe）值较低，接近飞地边缘。这些特征可归因于三个主要过程：（i）差分（±混合/混合）磁脉冲的集合; （ii）与糊状物中的密度流和残余熔体的偏析有关的局部流体力学分类; （iii）在上升过程中或与岩浆动力学有关的岩体内部，铁镁质岩浆的机械破坏和化学杂交。这些过程均不影响整个岩体，但仅限于特定的岩浆单元。因此，通过岩浆批次的增量组装进行的质子生长与岩浆室工艺并不矛盾。

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