We explore the origins of anorogenic post-breakup magmatism in two areas of the mid-Atlantic Appalachians: the New England-Quebec Province (ca. 130–120 Ma) and the Shenandoah Province (ca. 49–47 Ma). Radiometric rock ages and other data do not support claims that this magmatism occurred when these sites were located above postulated Great Meteor and Bermuda mantle hotspots/plumes. We propose instead that the sites are persistent lithospheric ‘weakspots’ favorable for magma ascent during relatively short intervals of a few Myr when global-scale plate motion reorganizes every 20–30 Myr. Magma ascends into the crust when compressive intra-plate stress is relaxed. Weakspots in the plate, not fixed mantle hotspots, can explain why anorogenic magmatism occurred at the same two sites also much earlier (by ca. 50 Myr in the New England-Quebec province and ca. 100 Myr in the Shenandoah Province), and why the Bermuda volcanoes formed not later, but coevally with the Shenandoah Province, 1400 km along the postulated hotspot trace. The plume hypothesis also fails to explain why the New England-Quebec magmas were emplaced at the same time as anomalously productive magmatism along the northern mid-Atlantic Ridge and coincident with the breakup of Iberia from the Grand Banks, sites almost 2000 km distant from the New England-Quebec Province. Moreover, New England-Quebec radiometric age distributions suggest that distant magmatic events and continental breakup affecting other plates were global plate reorganization events that may be ‘recorded’ by volcanism at weakspots. Shenandoah-Bermuda magmatism happened during the Pacific plate motion change recorded by the Hawaii-Emperor Bend. The ca. 720 Ma Robertson River Igneous Suite of anorogenic plutons in Virginia, USA, may be an old analog of the Shenandoah Province exploiting the same lithospheric weakspot. The New England-Quebec magmatic period 130–120 Ma is also the time over which the geomagnetic reversal frequency slowed, reaching zero at the onset of the Cretaceous normal superchron (C34n) at ca. 120 Ma. This event was recorded at the mid-Atlantic Ridge axis as the J-Anomaly Ridge and a large increase in spreading half-rate from 1 to 2.5 cm/a. Thus, geomagnetic reversal frequency may also be related to plate tectonics.

中文翻译：

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岩石圈弱点，而不是热点：全球板块构造、板块内应力和 LIP 背景下的新英格兰-魁北克和谢南多厄厌氧岩浆作用


我们探讨了大西洋中部阿巴拉契亚山脉两个地区裂解后岩浆作用的起源：新英格兰-魁北克省（约 130-120 马）和谢南多厄省（约 49-47 马）。辐射岩龄和其他数据并不支持这种岩浆作用发生在这些地点位于假定的大流星和百慕大地幔热点/羽流上方的说法。相反，我们提出这些地点是持续的岩石圈“弱点”，当全球尺度的板块运动每 20-30 Myr 重组一次时，这些地点是有利于岩浆在相对较短的几 Myr 间隔内上升的。当板内压缩应力松弛时，岩浆上升到地壳中。板块中的薄弱点，而不是固定的地幔热点，可以解释为什么同样两个地点的厌氧岩浆作用也发生得更早（新英格兰-魁北克省约为 50 Myr，Shenandoah 省约为 100 Myr），以及为什么百慕大火山不是晚于此，而是与雪兰多省同时代，沿着假定的热点轨迹 1400 公里。羽流假说也未能解释为什么新英格兰-魁北克岩浆与大西洋中北部的异常生产性岩浆同时出现，并且与伊比利亚从大浅滩分离同时发生，大浅滩距离新英格兰-魁北克省近 2000 公里。此外，新英格兰-魁北克辐射年龄分布表明，影响其他板块的遥远岩浆事件和大陆分裂是全球板块重组事件，可能被薄弱点的火山活动“记录”下来。Shenandoah-Bermuda 岩浆作用发生在 Hawaii-Emperor Bend 记录的太平洋板块运动变化期间。该 ca. 720 马 罗伯逊河 美国弗吉尼亚州的成岩组可能是雪兰多省利用同一岩石圈弱点的古老模拟。新英格兰-魁北克岩浆期 130-120 马 也是地磁反转频率减慢的时间，在白垩纪正常超时 （C34n） 开始时达到零，约为 120 马。该事件在大西洋海脊中轴被记录为 J 异常脊，并且扩散半速率从 1 cm/a 大幅增加到 2.5 cm/a。因此，地磁反转频率也可能与板块构造有关。