Palaeomagnetism and 40Ar/39Ar dating from Lower Jurassic rocks in Gastre, central Patagonia: further data to explore tectonomagmatic events associated with the break-up of Gondwana

New 40Ar/39Ar data indicate ages of c. 185 Ma for the Lonco Trapial volcanic field in Gastre, north–central Patagonia, implying that this andesitic unit is roughly coeval with the Marifil silicic province that crops out in the eastern part of northern Patagonia. These volcanic fields are therefore roughly coeval with the Karoo–Ferrar large igneous province, further contributing to the huge, 185–180 Ma magmatic outpouring in southern Gondwana. The comparison of palaeomagnetic results from the unit with Early Jurassic reference palaeopoles suggests that Gastre records a small, anticlockwise finite rotation with respect to stable South America. This contrasts sharply with the large clockwise rotation recorded by roughly coeval dolerites from the Falkland/Malvinas Islands, and argues against a direct relationship between the rotation of the islands and the tectonic activity in Gastre. The results support previous suggestions that the Gastre Fault System is not the locus of major strike-slip displacement activity in the Jurassic as some workers have suggested. Taken as a whole, palaeomagnetic data from Patagonia suggest a more complex than previously thought pattern of distributed and variable deformation during the early stages of Gondwana breakup. The crustal block boundaries in the region remain cryptic.

新的40Ar/39Ar测年数据显示，巴塔哥尼亚中北部加斯特雷（Gastre）地区的隆科·特拉皮亚尔（Lonco Trapial）火山岩场的形成年龄约为185 Ma，表明该安山岩单元与出露于北巴塔哥尼亚东部的马里菲尔（Marifil）硅质火成岩省大致同期。据此，上述火山岩场与卡鲁-费拉尔（Karoo–Ferrar）大火成岩省处于大致相同的地质时期，进一步佐证了冈瓦纳南部185~180 Ma期间大规模的岩浆喷发事件。将该单元的古地磁结果与早侏罗世参考古地磁极进行对比后发现，加斯特雷地区记录了相对于稳定南美板块的小型逆时针有限旋转。这与福克兰/马尔维纳斯（Falkland/Malvinas）群岛同期产出的辉绿岩所记录的大规模顺时针旋转形成鲜明反差，且不支持该群岛的构造旋转与加斯特雷地区的构造活动存在直接关联的推论。本研究结果支持此前的学术观点：侏罗纪时期的主要走滑位移活动并非如部分研究者所提出的那样，发生于加斯特雷断裂系统（Gastre Fault System）之上。综合来看，巴塔哥尼亚地区的古地磁数据表明，冈瓦纳裂解初期的弥散性与不均一变形模式比此前学界的认知更为复杂。该区域的地壳块体边界迄今仍属隐蔽未解。