Guzofski CA, Robinson AC, Tucker RD
J Geol. 2000 Mar;108(2):219-232. doi: 10.1086/314396.
New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian; (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.
新的铀铅年代学限定了新罕布什尔州北部缅因中央地体阿卡迪亚造山运动的时间。从六个样品中分析了十六份锆石或独居石，每份一至六颗：（1）一份记录阿卡迪亚运动开始的早期同构造闪长岩；（2）一份片岩、一份混合岩和两份花岗岩，它们共同记录了阿卡迪亚运动的高峰期；（3）一份记录阿卡迪亚运动结束的运动后深成岩体。同构造的瓦姆苏塔闪长岩中的锆石给出了约408 Ma的207Pb/206Pb年龄，此时变形造山楔与前陆盆地的边界位于总统山脉附近。这个年龄与西北迁移的阿卡迪亚造山前沿的艾姆斯期位置非常吻合，并记录了缅因中央地体这一部分阿卡迪亚运动的开始。我们提出了一个可能的阿卡迪亚构造模型，该模型整合了地质年代学、构造和地层数据。片岩、混合岩、比奇洛草坪花岗岩和斯莱德峰花岗岩中的独居石给出了207Pb/206U年龄，表明阿卡迪亚变质作用的高峰期和二云母花岗岩的侵入发生在约402 - 405 Ma，即阿卡迪亚造山运动的主要脉动期。先前报道的来自片岩的独居石年龄可能记录了新罕布什尔州缅因中央地体和缅因州西部的变质作用高峰期，范围约为406 - 384 Ma，新罕布什尔州东南部年龄较年轻，向西、北和东北方向年龄逐渐增大。总统山脉的阿卡迪亚造山运动在约355 Ma结束，这是皮博迪河花岗岩中独居石的207Pb/235U年龄。从408 Ma到可能至少394 Ma，总统山脉的阿卡迪亚造山运动是典型的构造样式，以同运动变质作用为主，在新罕布什尔州中部和南部、马萨诸塞州和康涅狄格州可见。从不早于394 Ma到最晚355 Ma，造山运动是缅因州部分地区典型的样式，以后运动变质作用为主。
