Barrovian metamorphism in nominally lowermost Alpujarride Complex units: tectonic implications for the development of the orogenic wedge in the Western Mediterranean

The sedimentary successions of the Permian-Triassic rifting related to the Alpine Tethys in the easternmost Lower Alpujarride units (Internal Betic Zone, south Spain) host shallow-intrusive basic bodies of Mesozoic age. These gabbroic bodies experienced low-grade Alpine metamorphic overprint, singularly recorded in three outcrops nearby Alicante city in southeast Spain: Callosa and Orihuela Ranges and Nueva Tabarca Island. Overall, the metabasic bodies still preserve magmatic ophitic/subophitic texture and relicts of magmatic augite and ferropargasite-ferroedenite-edenite and kaersutite. In contrast, metamorphic minerals include magnesioriebeckite, actinolite, albite, stilpnomelane, phengite, and chlorite. Major and trace element geochemical data suggest crystallization of the igneous protoliths from subalkaline (tholeiitic) to alkaline within-plate (continental) basaltic magmas slightly enriched in incompatible elements. These magmas with E-MORB geochemical affinity very likely intruded at shallow crustal levels in an extensional-synrift-geodynamic setting related to the breakoff of Pangea in the region. Pressure-Temperature (P-T) conditions estimated by average P-T method and pseudosection calculations suggest metamorphic peak at ca. 6 kbar at 340°C consistent with burial metamorphism down to ca. 18 km depth with intermediate thermal gradients of ca. 18°C/km. These conditions are different to the typical subduction-related blueschist facies conditions of the Lower Alpujarride units and suggest, instead, a collisional (Barrovian) event. This can be explained by a late-stage accretion of the studied units after the main subduction-related event, pointing to the dispersion of the corresponding Lower Alpujarride sections along the margin in different palaeogeographic positions before deformation and movement to their actual structural position. The metamorphic correlation of the studied units with the accreted passive margin of South Iberia (Subbetic zone) is also discussed.

西班牙南部内贝蒂奇带（Internal Betic Zone）最东端的下阿尔胡拉赖德地层单元（Lower Alpujarride units）中，与阿尔卑斯特提斯（Alpine Tethys）相关的二叠纪-三叠纪沉积序列，赋存了中生代时期的浅侵入基性岩体。这些辉长岩体遭受了低级阿尔卑斯期变质叠加作用，其独特的变质信号仅见于西班牙东南部阿利坎特市周边三处露头：卡略萨与奥里韦拉山脉，以及新塔巴尔卡岛。总体而言，该变基性岩体仍保留了岩浆间粒/半间粒结构（ophitic/subophitic texture），以及岩浆普通辉石、铁韭闪石-铁浅闪石-浅闪石与钛角闪石的残余组分。与之相对，其变质矿物组合包括镁钠闪石、阳起石、钠长石、黑硬绿泥石、多硅白云母与绿泥石。主量与微量元素地球化学数据表明，其火成原岩源自亚碱性（拉斑玄武质）至碱性的板内（大陆）玄武质岩浆，且这类岩浆略富集不相容元素。这类具有富集型洋中脊玄武岩（E-MORB）地球化学亲和性的岩浆，极有可能在与该区域泛大陆裂解相关的伸展同裂谷地球动力学背景下，侵入浅地壳深度。通过平均P-T法与相图（pseudosection）计算估算得到的压力-温度（P-T）条件显示，变质峰期约为340℃、6千巴，与埋藏至约18千米深度的埋藏变质作用一致，对应中等地热梯度约18℃/km。该条件不同于下阿尔胡拉赖德地层单元典型的俯冲相关蓝片岩相变质条件，反而指示了一次碰撞（巴罗式）事件。这可通过研究地层单元在主俯冲事件之后的晚期增生作用来解释，表明在变形并迁移至现今构造位置之前，对应的下阿尔胡拉赖德地层剖面沿陆缘分散于不同古地理位置。本文还探讨了本次研究的地层单元与南伊比利亚亚被动陆缘（苏贝蒂克带，Subbetic zone）的变质对比关系。