Multiple early Paleozoic volcanic events at the northern Gondwana margin: U–Pb age evidence from the Southern Variscan branch (Sardinia, Italy)

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• 作者：Oggiano Giacomo ; Gaggero Laura ; Funedda Antonio ; Buzzi Laura ; Tiepolo Massimo
• 关键词：Cambrian Ordovician ; Sardic phase ; Volcanism ; U– ; Pb zircon geochronology ; North Gondwana margin ; Sardinia
• 刊名：Gondwana Research
• 出版年：2010
• 出版时间：January 2010
• 年：2010
• 卷：17
• 期：1
• 页码：44-58
• 全文大小：1672 K

文摘

In Sardinia, one of the southernmost remains of the European Variscan belt, a crustal section through different Gondwanan paleodomains is largely preserved. Laser ablation ICP-MS U–Pb ages on undoubtedly igneous sites of the zircons were determined on the Lower Palaeozoic volcanic rocks, constrained by defined field relationships, thus evidencing three subsequent volcanic events:

I. Intermediate and felsic (491.7 ± 3.5 Ma ÷ 479.9 ± 2.1 Concordia ages) transitional volcanic rocks embedded within a Cambro–Ordovician terrigenous succession, that occurs with continuity in external and inner nappes, bounded to the top by the Sardic unconformity.

II. This Cambrian–Lower Ordovician succession is cut by calc-alkalic rhyodacites, which yielded a Concordia age of 465.4 ± 1.4 Ma, confirming their pertinence to the huge, bimodal Mid-Ordovician arc volcanism, commonly interpreted as the widespread marker of the Rheic ocean subduction.

III. Alkalic metaepiclastites in the external nappe, within the post-Caradocian transgressive sequence, dated at 440 ± 1.7 Ma, likely related to rifting and collapse of the Mid-Ordovician volcanic arc.

In the reshaped Lower Palaeozoic stratigraphy of Sardinia, the timing of the early steps of the Variscan Wilson cycle can be inferred.