Petrological evolution of the Middle Triassic Predazzo Intrusive Complex, Italian Alps

The Predazzo Intrusive Complex (PIC), a Ladinian plutonic body located in the Southern Alps (NE Italy), is made up of a 4.5 km3 gabbroic to syenitic and syenogranitic intrusion, basaltic to latitic volcanic products (about 6 km3 in volume) and by an extended dike swarm intruding both intrusive and volcanic rocks. An extensive field survey of the complex, followed by detailed petrographic and geochemical analyses, allowed the identification of three different magmatic units: a Shoshonitic Silica Saturated Unit (SS), 3.1 km3 in volume, a Shoshonitic Silica Undersaturated Unit (SU), 0.3 km3 in volume, and a Granitic Unit (GU), 1.1 km3 in volume. K-affinity, marked Nb and Ti negative anomalies and a strong Pb enrichment are distinctive markers for all PIC lithotypes. A general HFSE (Th, U, Pb), LREE (La, Ce, Pr, Nd) and Na enrichment characterizes the SU suite with respect to the SS series. Mass balance calculations, based on major and trace element whole rock and mineral compositions, have been used to simulate the fractionation process of SS and SU suites, showing (i) the complexity of the evolutionary stages of the PIC and (ii) the analogy between the calculated subtracted solid assemblages and the natural cumulitic lithotypes outcropping in the area. The field relationships between the various portions of the intrusive complex, the volcanic products and the dike swarm define the temporal evolution of the PIC, in which the SS magma batch was followed by the GU and later on by the SU intrusion. The presence, in both eastern and western portions of the complex, of a transitional magmatic contact between the intrusive rocks of the SS suite and the volcanics is not in favour of the hypothesis of a caldera collapse to explain the ring-like shape of the PIC.

佩拉佐侵入杂岩体（Predazzo Intrusive Complex, PIC）是一处赋存于意大利东北部南阿尔卑斯山的拉丁期深成岩体，由体积达4.5 km³的辉长岩-正长岩-正长花岗岩侵入体、体积约6 km³的玄武岩至安粗岩火山产物，以及侵入于侵入岩与火山岩中的大范围岩脉群组成。研究团队对该杂岩体开展了全面的野外调查，并辅以详细的岩石学与地球化学分析，最终识别出三个不同的岩浆单元：体积3.1 km³的橄榄安粗岩系硅饱和单元（Shoshonitic Silica Saturated Unit, SS）、体积0.3 km³的橄榄安粗岩系硅不饱和单元（Shoshonitic Silica Undersaturated Unit, SU），以及体积1.1 km³的花岗岩单元（Granitic Unit, GU）。所有PIC岩石类型均以钾亲和性、显著的铌与钛负异常，以及强烈的铅富集为典型特征；相较于SS系列，SU套系具有更高的高场强元素（HFSE，包括Th、U、Pb）、轻稀土元素（LREE，包括La、Ce、Pr、Nd）与钠富集程度。研究基于全岩及矿物的主量与微量元素组成开展质量平衡计算，以此模拟SS与SU套系的分异演化过程，结果表明：其一，PIC的岩浆演化阶段具有复杂性；其二，计算得到的分离固相组合与区域内出露的天然堆晶岩类具有相似性。侵入杂岩体各部分、火山产物与岩脉群之间的野外接触关系限定了PIC的时间演化序列：SS岩浆批次之后依次为GU侵入体，随后是SU侵入体。在该杂岩体的东西两侧均可见SS套系侵入岩与火山岩之间的过渡岩浆接触带，这一现象并不支持以破火山口塌陷来解释PIC环状形态的假说。