Pre-Alpine prograde evolution of the Upper Alpujarride (Betic-Rif belt) reveals a Paleotethys-related collision

Medium and high grade kyanite-bearing graphitic metapelites of the basement of the Torrox Unit (Upper Alpujarride, Western Mediterranean) show kyanite-andalusite-kyanite replacements and coexisting garnet types with complex compositional zoning and growth-dissolution stages. This allows inferring two clockwise P-T paths associated with contrasted orogenic cycles of latest Carboniferous – earliest Permian and Eocene(?)-Miocene age, respectively. Dated (306 ± 77 Ma) prograde medium grade garnet cores with relatively low to moderate Ca content indicates pre-Alpine barrovian metamorphism in the kyanite stability field (ca. 600°C, 0.7–0.8 GPa). This implies a prograde pressurization of the sequence followed by decompression, as inferred from andalusite replacements after kyanite. Deeper in the sequence, anatectic granitic orthogneisses contain high-grade graphitic kyanite-bearing metapelitic restitic enclaves. Dating of rutile (with inclusions of kyanite) and monazite (shielded in biotite) from the pelitic enclaves provided pre-Alpine ages (ca. 300 Ma) rejuvenated due to Alpine overprint. Such pressurization event is not consistent with a late Variscan stage, commonly identified with a tectonic scenario characterized by orogenic collapse and decompression at ca. 300 Ma. Instead, we propose a post-Variscan collision event related to the closure of the western Paleotethys. On the other hand, Ca-rich cores of garnet porphyroblasts in the medium-grade metapelites formed at 1.1–1.2 GPa, 500°C, implying a metamorphic gradient of ca. 11.5°C/km related to Eocene (?) subduction. Kyanite+phengite replacements after andalusite are related to this stage. Late Mg-rich Ca-poorer overgrowths in garnet porphyroblasts document increasing temperature during the first stage of decompression (down to 0.9 GPa at ca. 650°C) with increasing thermal gradient up to ca. 20°C/km. These two complete metamorphic cycles of pressurization and heating followed by decompression allow reconciling previous contradictory conclusions about the paragenetic and thermal gradient development during the Paleotethyan and Alpine cycles in different units of the Betic-Rif belt.

西地中海地区上阿尔普哈拉德地体（Upper Alpujarride）托罗克斯单元（Torrox Unit）基底的中高级含蓝晶石（kyanite）石墨泥质变质岩，发育蓝晶石-红柱石（andalusite）-蓝晶石的交代组构，以及兼具复杂成分环带与生长-溶解阶段的共生石榴子石（garnet）变种。据此可推断出两条顺时针压力-温度（P-T）路径，二者分别对应晚石炭世末期-早二叠世、始新世（？）-中新世两个截然不同的造山旋回。定年结果显示，（306±77 Ma）的进变质中级石榴子石核部具有相对低至中等的Ca含量，表明蓝晶石稳定域内（约600℃、0.7~0.8 GPa）存在阿尔卑斯前巴罗式（barrovian）变质作用。该结果指示该岩层序列曾经历进变质增压作用，随后发生减压，这一推论可由红柱石交代蓝晶石的组构得到佐证。在岩层序列更深部，深熔花岗正片麻岩（anatectic granitic orthogneisses）中赋存高级含石墨蓝晶石泥质变质岩残留包体（restitic enclaves）。对该泥质包体中含蓝晶石包裹体的金红石（rutile），以及被黑云母（biotite）包裹的独居石（monazite）进行定年，得到了受阿尔卑斯叠加改造而年轻化的阿尔卑斯前变质年龄（约300 Ma）。此类增压事件与通常认为的约300 Ma时以造山垮塌和减压为特征的构造背景下的晚华力西（Variscan）期构造事件并不相符，据此我们提出该增压事件与古特提斯洋（Paleotethys）西段闭合相关的华力西后碰撞事件有关。另一方面，中级泥质变质岩中的富钙石榴子石变斑晶（garnet porphyroblasts）核部形成于1.1~1.2 GPa、500℃的条件下，对应约11.5℃/km的变质热梯度，与始新世（？）俯冲作用相关。红柱石被蓝晶石+多硅白云母（phengite）交代的组构即形成于该阶段。石榴子石变斑晶晚期发育富镁、贫钙的增生边，记录了减压第一阶段（约650℃时降至0.9 GPa）的升温过程，同期热梯度最高可达约20℃/km。这两个完整的增压-加热再到减压的变质旋回，可为解决贝蒂科-里夫造山带（Betic-Rif belt）不同地体中，古特提斯与阿尔卑斯旋回期间的共生组合与热梯度演化这一此前存在矛盾的结论提供依据。