Petrogenesis of the meta-igneous rocks of the Sierra El Arco and coeval magmatic rocks in Baja California: Middle Jurassic-Early Cretaceous (166-140 Ma) island arc magmatism of NW México

The Jurassic-Early Cretaceous Vizcaíno and Alisitos terranes are juxtaposed in the central part of the Baja California Peninsula, México. The southernmost outcrops of the Alisitos terrane are located in the Sierra El Arco, which consists of greenschist-facies meta-igneous rocks. Further east in the Calmallí area, amphibolite and paragneiss dominate. The meta-igneous rocks constitute a magmatic system ranging from peridotite to granodiorite, with associated basaltic to dacitic volcanic rocks. The suite displays negative εSrt (average −8.4, n = 23) and positive εNdt values (average +5.8, n = 23), suggesting mantle-derived melts with no discernible assimilation of continental crustal material. Geochemical and isotopic data of the amphibolite suggests a protolith similar to the meta-igneous rocks. Paragneiss samples display εSrt of +166 and εNdt around −7, with Nd crustal residence ages of 1.5 Ga. The isotopic data of the paragneiss suggest provenance from continental sources. Similar to Triassic-Middle Jurassic metasedimentary rocks from the Baja California Peninsula and western México. Middle Jurassic magmatism on Cedros Island (Vizcaíno terrane) and the Sierra El Arco (Alisitos terrane) define an εSrt vs. εNdt evolutionary trend that is mostly within the mantle array, suggesting a common origin within the same island arc subduction setting. Jurassic magmatism of continental affinity, defined by positive εSrt (average +39, n = 15) and negative εNdt values (average −4.2, n = 15), is widespread in the Baja California Peninsula and western México. Our Sr-Nd isotope mixing curves suggest that the assimilation of Late Triassic metasedimentary rocks or a lower crust could explain the isotopic composition of the continental affinity Jurassic magmatism.

墨西哥下加利福尼亚半岛中部，侏罗纪-早白垩世的维萨卡尼奥地体（Vizcaíno terrane）与阿利西托斯地体（Alisitos terrane）呈叠置接触关系。阿利西托斯地体最南端的露头分布于埃尔阿尔科山（Sierra El Arco），该区域的岩石组合以绿片岩相变火成岩为主。向东延伸的卡马利（Calmallí）地区，则以角闪岩与副片麻岩为主要岩石类型。这套变火成岩构成了一套从橄榄岩至花岗闪长岩的完整岩浆系统，伴生有玄武质至英安质的火山岩序列。该岩石组合呈现负的时间校正初始锶同位素εSr(t)值（平均值为-8.4，样本量n=23）与正的时间校正初始钕同位素εNd(t)值（平均值为+5.8，n=23），指示其为幔源熔体，未受到可识别的大陆地壳物质混染。角闪岩的地球化学与同位素数据显示，其原岩与前述变火成岩具有相似性。副片麻岩样品的εSr(t)值为+166，εNd(t)值约为-7，其钕同位素地壳居留年龄为1.5 Ga。副片麻岩的同位素数据表明其物源来自大陆地壳，与下加利福尼亚半岛及墨西哥西部的三叠纪-中侏罗世变沉积岩特征一致。塞德罗斯岛（维萨卡尼奥地体）与埃尔阿尔科山（阿利西托斯地体）的中侏罗世岩浆作用，呈现出εSr(t)-εNd(t)演化趋势，该趋势基本落入幔源同位素阵列范围内，指示二者形成于同一岛弧俯冲环境，具有共同成因。具有大陆亲缘性的侏罗纪岩浆作用在墨西哥下加利福尼亚半岛及西部广泛分布，其同位素特征表现为正的εSr(t)值（平均值为+39，样本量n=15）与负的εNd(t)值（平均值为-4.2，n=15）。本研究的Sr-Nd同位素混合曲线模拟结果表明，晚三叠世变沉积岩或下地壳的混染作用，可合理解释该类具有大陆亲缘性侏罗纪岩浆作用的同位素组成。