Slab window basalt cycles in extra-Andean Patagonia formed in a main-plateau and a post-plateau stage, during glacial and interglacial events. Here, we provide field, geochemical and geochronological data of Pliocene basalts and volcanoes from the north of Viedma lake (49° 20′–49° 30′ S). These basalts share the alkaline composition and enrichment in incompatible elements of post-plateau lavas, in agreement with their radiometric ages. While most post-plateau basalts were erupted through scoria cones, we found that some effusions to the west built lava shields of scutulum type. Field evidence indicates that shields formed during or soon after deglaciation, whereas scoria cones formed before ice retreat or at interglacial stages. We obtained an Ar/Ar age of 3.59 ± 0.07 Ma for a pre-shield basalt, extruded immediately before or concomitant with till deposition. Lava shield formation occurred soon after this age. The lava shield basalts are enriched in incompatible elements. Their petrogenesis involved low degree (~ 2.5%) partial melting of a garnet lherzolite source at the waning of the post-plateau stage, followed by stagnation and high pressure fractionation. Subsequent low pressure fractionation also occurred during ascent. Pre-shield basalts evolved by slightly greater degree of partial melting (~ 3–3.5%) and lesser fractionation. We propose that during glaciation, glacial loading favoured magma ponding, probably at the base of the crust, and that during/after deglaciation lithospheric relaxation facilitated tapping of stored, more evolved magmas. Stagnation favoured outgassing, so that low ascent and eruption rates of outgassed magma triggered a change to an effusive style.

在冰川和间冰期事件期间，在主高原和后高原阶段形成了安第斯巴塔哥尼亚外的板窗玄武岩循环。在这里，我们提供了 Viedma 湖北部（49° 20'–49° 30' S）上新世玄武岩和火山的野外、地球化学和地质年代学数据。这些玄武岩具有碱性成分，富含高原后熔岩的不相容元素，与其辐射年龄一致。虽然大多数高原后玄武岩都是通过火山渣锥喷发的，但我们发现西边的一些渗出液形成了盾片类型的熔岩盾牌。现场证据表明，在冰川消退期间或之后不久形成了盾牌，而在冰退之前或间冰期阶段形成了火山渣锥。对于预屏蔽玄武岩，我们获得了 3.59 ± 0.07 Ma 的 Ar/Ar 年龄，在沉积之前或同时挤压。在这个时代之后不久，熔岩盾就形成了。熔岩盾玄武岩富含不相容的元素。它们的岩石成因涉及在后期高原阶段减弱时石榴石二镁橄榄石源的低度（~2.5%）部分熔融，然后是停滞和高压分馏。随后的低压分馏也在上升过程中发生。Pre-shield 玄武岩通过稍微更大程度的部分熔化 (~ 3-3.5%) 和更少的分馏而演变。我们认为，在冰川作用期间，冰川载荷有利于岩浆积聚，可能在地壳底部，并且在冰川作用期间/之后，岩石圈松弛促进了储存的、更演化的岩浆的开采。停滞有利于排气，