Collaborative Research: Stratigraphic Resolution as Key to Tectonic Synthesis in the Southeastern New England Avalon Zone Around Boston, Massachusetts

由于新的同位素数据揭示了长期被接受的区域地层学中的根本缺陷，新的同位素数据揭示了长期被接受的区域地层学中的根本缺陷，因此有必要在新英格兰东南部阿瓦隆带的波士顿部分进行全面的古地磁/年代学研究。例如：1.据报道，罗克斯伯里砾岩中四个地理上分离的剖面中的布莱顿火山夹层的最大年龄约为587 Ma、490 Ma、438 Ma和362 Ma，这是根据1-4个颗粒锆石组分得出的。据报道，奥陶系昆西花岗岩(基于437+23 Ma的上截年龄)已经产生了硅罗-泥盆纪锆石(单颗粒分析，207Pb/206Pb年龄约为410 Ma)。据报道，波士顿盆地以西的630+15 Ma Dedham花岗岩基于三个单一的锆石分析，获得了约610 Ma的加权平均207Pb/206Pb年龄。基于这些年代学数据的初步古极进一步表明：1.以前报道的形成罗克斯伯里砾岩的新元古代阿瓦罗纪极来自志留系或更年轻的岩石。可靠的新元古代岩石形成于中低纬度，而不是之前报道的中高纬度，时间间隔为610至~570 Ma。这项提议寻求资金，以继续澄清波士顿地区的地层和构造关系，以此为基础完成5到6个测年良好的古地磁极。这些极点将制约对新元古代弧状岩浆作用期间新英格兰东南部阿瓦隆带古纬度的修订解释、其在假想的“雪球”地球气候事件期间的位置以及中古生代向北美吸积的时间。定向古地磁岩芯将在21个新的地点收集，包括阿瓦隆弧岩中的Dedham花岗岩和Mattapan火山杂岩，上覆的新元古代地层中包括剑桥Argillite和冰晶矿床，包括Roxbury砾岩中的SQuantum成员，在“Bright ton”火山中，以及在其他几个年龄不确定或约束较差的地层中(据报道，包括Medford堤坝和小牛岛礁的侏罗纪侵入岩)。其中12个地点也是支持年代学的目标，10个地点是地球化学的目标。

Integrated paleomagnetic/geochronologic study necessitates radical tectonic re-thinking in the Boston portion of the Southeastern New England Avalon Zone because new isotopic data reveal fundamental flaws in the long accepted regional stratigraphy. For example:1. Reportedly Neoproterozoic, but previously undated Brighton volcanic interbeds from four geographically separated sections in the Roxbury Conglomerate have maximum ages of approximately 587 Ma, 490 Ma, 438 Ma and 362 Ma based on 1-4 grain zircon fractions.2. Reportedly Ordovician Quincy Granite (based on an upper intercept date of 437 + 23 Ma) has yielded Siluro-Devonian zircons (single grain analyses with 207 Pb/ 206 Pb dates of approximately 410 Ma).3. Reportedly 630 + 15 Ma Dedham Granite west of the Boston Basin has yielded a weighted mean 207 Pb/ 206 Pb date of approximately 610 Ma based on three single zircon analyses.Preliminary paleopoles based on these geochronological data further suggest that:1. The previously reported Neoproterozoic Avalonian pole form the Roxbury Conglomerate is from Silurian or younger rocks.2. Reliably dated Neoproterozoic rocks formed at low-mid latitudes rather than the previously reported high-mid latitude for the 610 to ~ 570 Ma interval.This proposal seeks funding for continued work to clarify stratigraphic and structural relationships inthe Boston area as a basis for completing 5 or 6 well dated paleomagnetic poles. These poles will constrain revised interpretations of the paleolatitude of the Southeastern New England Avalon Zone during Neoproterozoic arc magmatism, its position during during hypothesized "snowball" earth climatic events and the timing of mid-Paleozoic accretion to North America. Oriented paleomagnetic cores will be collected at 21 new locations in Avalonian arc rocks including Dedham Granite and Mattapan Volcanic Complex, in overlying Neoproterozoic strata including the Cambridge Argillite and glaciomarine deposits comprising the Squantum Member of the Roxbury Conglomerate, in "Brighton" Volcanics and in several other formations of uncertain or poorly constrained age (including reportedly Jurassic intrusive rocks of the Medford dike and Calf Island sill). Twelve of these sites are also targeted for supporting geochronology and ten for geochemistry.