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

综合古地磁/地质年代学研究需要对新英格兰东南部阿瓦隆带波士顿部分进行彻底的构造反思，因为新的同位素数据揭示了长期以来公认的区域地层学的根本缺陷。例如:1。据报道，在罗克斯伯里砾岩的四个地理上分开的区段中，有新元古代的布里顿火山互层，但以前没有确定年代，根据1-4粒锆石的分值，最大年龄约为587 Ma， 490 Ma， 438 Ma和362 Ma。据报道，奥陶系昆西花岗岩（基于上部截取日期为437 + 23 Ma）发现了矽泥盆系锆石（单粒分析，207pb / 206pb日期约为410 Ma）。据报道，波士顿盆地以西630 + 15 Ma的Dedham花岗岩在三次锆石分析的基础上得出了加权平均207 Pb/ 206 Pb日期，约为610 Ma。基于这些地质年代学资料的初步古极进一步表明：先前报道的罗克斯伯里砾岩的新元古代阿瓦洛尼亚极来自志留纪或更年轻的岩石。在610 ~ ~ 570 Ma区间，新元古代岩石形成于中低纬度，而不是先前报道的中高纬度。该提案寻求资助继续澄清波士顿地区地层和构造关系的工作，作为完成5或6个确定年代的古磁极的基础。这些极点将限制对新元古代弧岩浆活动期间新英格兰东南部阿瓦隆带古纬度的修正解释，它在假设的“雪球”地球气候事件期间的位置以及中古生代向北美增生的时间。定向古磁芯将在Avalonian弧岩（包括Dedham花岗岩和Mattapan火山杂岩）的21个新位置收集，在新元古代地层（包括Cambridge argilite和glaciomine矿床，包括Roxbury砾岩的Squantum成员）的上覆地层中，在“Brighton”火山和其他几个年龄不确定或约束不佳的地层中（包括据报道的Medford岩壁和Calf Island的侏罗纪侵入岩）。其中12个地点也是支持地质年代学的目标，10个地点是支持地球化学的目标。

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.