Early Cambrian alkaline volcanism on the southern margin of Laurentia: evidence in the volcaniclastic units from the Puerto Blanco Formation in the Caborca block, NW Mexico

Volcaniclastic units are exposed at the base of the Puerto Blanco Formation in the Caborca region, northwestern Mexico. The lower unit reveals the presence of Early Cambrian mafic volcanism in this region. It consists of a volcano-sedimentary sequence represented by tuffaceous conglomerates, agglomerates, lapillistones, tuffs, and altered mafic volcanic flows. Petrographic analysis classified the volcanic clasts as albite-sphene-calcite-actinolite granofels, with a moderate to intense hydrothermal alteration, precisely characterized by EPMA analysis. Albite-actinolite geothermometry indicates temperatures from 400 to 500°C, suggesting metamorphic conditions in the upper temperature greenschist facies. Geochemistry analysis shows a high TiO2 basic–ultrabasic volcanism that originated the volcanic clasts. Rock protoliths were studied using immobile trace elements, which classified them as OIB-type alkaline basalts with the characteristic spider hump-shaped pattern, situated in an anorogenic intracontinental tectonic setting with enriched mantle signatures. 40Ar/39Ar geochronology shows metamorphic ages of 52.58 ± 2.0 and 91.67 ± 0.55 Ma, consistent with the emplacement of Laramidic granitoids identified in the region. Possible correlations of this alkaline volcanism include the Southern Oklahoma Aulacogen and the late stages of the rifting of north western Laurentia represented in western United States.

墨西哥西北部卡博尔卡（Caborca）地区的普埃尔托布兰卡组（Puerto Blanco Formation）底部出露火山碎屑岩单元（volcaniclastic units）。这套火山碎屑岩的下部亚单元揭示了本区早寒武世基性火山作用的存在，其由一套以凝灰质砾岩、集块岩、火山砾岩（lapillistones）、凝灰岩及蚀变基性熔岩流为代表的火山沉积序列构成。岩石学分析（Petrographic analysis）将该序列中的火山碎屑归为钠长石-榍石-方解石-阳起石粒变岩（granofels），其普遍遭受中等至强烈的热液蚀变（hydrothermal alteration），该蚀变特征经电子探针显微分析（EPMA, Electron Probe Micro-Analysis）得以精准表征。钠长石-阳起石地质温压计（Albite-actinolite geothermometry）测算得到的变质温度区间为400~500℃，指示其变质条件处于高级绿片岩相（upper temperature greenschist facies）。地球化学分析（Geochemistry analysis）显示，形成该火山碎屑的火山作用属于高TiO₂基性-超基性火山作用。通过不活动微量元素（immobile trace elements）对火山碎屑原岩（Rock protoliths）进行判别，可将其归为洋岛玄武岩（OIB, Ocean Island Basalt）型碱性玄武岩，该类玄武岩具有典型的驼峰状微量元素蛛网图（spider hump-shaped pattern）特征，形成于非造山陆内构造环境（anorogenic intracontinental tectonic setting），且具备富集地幔特征（enriched mantle signatures）。⁴⁰Ar/³⁹Ar年代学（⁴⁰Ar/³⁹Ar geochronology）测试获得的变质年龄分别为52.58±2.0 Ma和91.67±0.55 Ma，与本区已识别的拉拉米德型花岗岩类（Laramidic granitoids）的侵位时代一致。该碱性火山作用可与俄克拉荷马南部拗拉槽（Southern Oklahoma Aulacogen）以及以美国西部为代表的劳伦大陆（Laurentia）西北缘裂谷作用晚期产物进行对比。