USGS Science⌕ Search

Geology topics

John N. Aleinikoff

Publications and source records attributed to John N. Aleinikoff.

At least 37 records · Page 2Linked to original sources
USGS2014

Age, chemistry, and correlations of Neoproterozoic–Devonian igneous rocks of the Arctic Alaska–Chukotka terrane: An overview with new U-Pb ages

The Arctic Alaska–Chukotka terrane is a microcontinent with an origin exotic to Laurentia. We used a sensitive high-resolution ion microprobe (SHRIMP) to date nine samples of Neoproterozoic rock and five samples of Devonian rock from the Brooks Range and Seward Peninsula of Alaska and from the Chukotka Peninsula of northeastern Russia. Felsic magmatism occurred at 968 Ma and 742 Ma in the Brooks Range and at 865 Ma and 670–666 Ma on Seward Peninsula. Felsic igneous rocks in Chukotka were dated at 656 Ma and 574 Ma. Devonian igneous rocks are found throughout the Arctic Alaska–Chukotka terrane, and we dated samples with ages of 391 Ma, 390 Ma, 385 Ma, 371 Ma, and 363 Ma. The felsic character of the Neoproterozoic rocks suggests formation at least in part through crustal melting. The age of the crustal source rocks that melted to form the Neoproterozoic rocks is inferred to be Mesoproterozoic based on Nd model ages ranging from 1.6 to 1.4 Ga. Rocks of this age range have been reported from the basement of Baltica but are rare in Laurentia. The 565 Ma orthogneisses on Seward Peninsula have ca. 1.1 Ga Nd model ages. Devonian igneous rocks have a wide range of model ages ranging from 1.6 to 0.8 Ga. The tectonic setting of the 968 Ma, 865 Ma, and 742 Ma rocks is unknown. The ca. 670 Ma magmatism on Seward Peninsula is interpreted to have occurred in an arc setting based on geochemistry and similarities in their ages to the Avalonian–Cadomian arc system peripheral to Gondwana. Latest Neoproterozoic magmatism is inferred to have occurred in a rift setting based on composition and the Paleozoic passive margin sequence that was deposited across the Arctic Alaska–Chukokta terrane. Devonian magmatism likely occurred in an arc and/or backarc rift setting. Significant uncertainties remain concerning the age of the Arctic Alaska–Chukotka terrane basement, particularly the age of the host rocks for Neoproterozoic intrusions.

Alaska↗
USGS2014

U-Pb detrital zircon geochronology as evidence for the origin of the Nome Complex, northern Alaska, and implications for regional and trans-Arctic correlations

Detrital zircons from the Nome Complex, a metamorphic terrane in northern Alaska, reveal important constraints on the early Paleozoic history of the Arctic Alaska–Chukotka terrane, a microcontinental block with an origin exotic to Laurentia. Twenty-two samples (17 in this study, five previously published) produce three detrital zircon population patterns (called themes), indicating that at least three distinguishable source areas contributed to the metamorphic protolith. Detrital zircon populations from metamorphosed rift-related mafic volcaniclastic rocks, a lithologic subunit of the Nome Complex, contain a dominant population of 740–550 Ma zircons. Samples from three other lithologic units yielded populations dominated by early Paleozoic zircons and characterized by a large population of 450–420 Ma zircons. A few samples, taken from two different lithologic units, yielded populations dominated by Mesoproterozoic zircons (most around 1.25–0.9 Ga) and lacked zircons younger than 900 Ma. None of the 22 samples contained more than a few Archean zircons. The ages of the youngest detrital zircon populations indicate that little of the protolith for the Nome Complex can be as old as Proterozoic, as previously thought. Further, a significant part of the protolith sequence is Devonian or younger; these rocks are likely correlative with Devonian or Mississippian units in the Brooks Range, specifically marine parts of the Endicott or Lisburne Groups. Based on detrital zircon data, limiting factors can be placed on the paleogeographic history of the Nome Complex and associated parts of the Arctic Alaska–Chukotka terrane: (1) 740–550 Ma zircons were deposited in a rift-related basin formed on a continental margin in the early Paleozoic; at least some of those zircons may have been sourced from local basement; (2) a transition to new sediment sources is reflected in Devonian or younger protoliths with the appearance of 450–420 Ma and 1.25–0.9 Ga detrital zircons; and (3) 450–420 Ma and 1.25–0.9 Ga zircons may have been supplied from sources outside the Arctic Alaska–Chukotka terrane.

Alaska↗
USGS2014

Geologic map of the Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska

This map was developed by the U.S. Geological Survey Mineral Resources Program to depict the fundamental geologic features for the western part of the Fortymile mining district of east-central Alaska, and to delineate the location of known bedrock mineral prospects and their relationship to rock types and structural features. This geospatial map database presents a 1:63,360-scale geologic map for the Kechumstuk fault zone and surrounding area, which lies 55 km northwest of Chicken, Alaska. The Kechumstuk fault zone is a northeast-trending zone of faults that transects the crystalline basement rocks of the Yukon-Tanana Upland of the western part of the Fortymile mining district. The crystalline basement rocks include Paleozoic metasedimentary and metaigneous rocks as well as granitoid intrusions of Triassic, Jurassic, and Cretaceous age. The geologic units represented by polygons in this dataset are based on new geologic mapping and geochronological data coupled with an interpretation of regional and new geophysical data collected by the Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys. The geochronological data are reported in the accompanying geologic map text and represent new U-Pb dates on zircons collected from the igneous and metaigneous units within the map area.

Alaska↗
USGS2013

Implications for late Grenvillian (Rigolet phase) construction of Rodinia using new U-Pb data from the Mars Hill terrane, Tennessee and North Carolina, United States

New data for zircon (external morphology, cathodoluminescence zoning, and sensitive high resolution ion microprobe [SHRIMP] U-Pb ages) from the Carvers Gap granulite gneiss of the Mars Hill terrane (Tennessee and North Carolina, United States) require reevaluation of interpretations of the age and origin of this rock. The new results indicate that the zircon is detrital and that the sedimentary protolith of this gneiss (and related Cloudland gneiss) was deposited no earlier than ca. 1.02 Ga and was metamorphosed at ca. 0.98 Ga. Tectonic models that included the gneiss as a piece of 1.8 Ga Amazonian crust (perhaps as part of the hypothetical Columbia supercontinent) are now untenable. The remarkably fast cycle of exhumation, erosion, deposition, and deep burial also is characteristic of other late Grenvillian (post-Ottawan) Mesoproterozoic paragneisses that occur throughout the Appalachians. These rocks provide new evidence for the duration of the formation of the Rodinia supercontinent lasting until at least 0.98 Ma.

North Carolina, Tennesee↗
USGS2013

Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

Alaska, British Columbia, Yukon↗
USGS2013

Bedrock geologic map of the Nashua South quadrangle, Hillsborough County, New Hampshire, and Middlesex County, Massachusetts

The bedrock geology of the 7.5-minute Nashua South quadrangle consists primarily of deformed Silurian metasedimentary rocks of the Berwick Formation. The metasedimentary rocks are intruded by a Late Silurian to Early Devonian diorite-gabbro suite, Devonian rocks of the Ayer Granodiorite, Devonian granitic rocks of the New Hampshire Plutonic Suite including pegmatite and the Chelmsford Granite, and Jurassic diabase dikes. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing hydrogeologic characterization of the fractured bedrock of Massachusetts and New Hampshire. This report presents mapping by G.J. Walsh and R.H. Jahns and zircon U-Pb geochronology by J.N. Aleinikoff. The complete report consists of a map, text pamphlet, and GIS database. The map and text pamphlet are only available as downloadable files (see frame at right). The GIS database is available for download in ESRI TM shapefile and Google Earth TM formats, and includes contacts of bedrock geologic units, faults, outcrops, structural geologic information, photographs, and a three-dimensional model.

Massachusetts, New Hampshire↗
USGS2012

Igneous activity, metamorphism, and deformation in the Mount Rogers area of SW Virginia and NW North Carolina: A geologic record of Precambrian tectonic evolution of the southern Blue Ridge Province

Mesoproterozoic basement in the vicinity of Mount Rogers is characterized by considerable lithologic variability, including major map units composed of gneiss, amphibolite, migmatite, meta-quartz monzodiorite and various types of granitoid. SHRIMP U-Pb geochronology and field mapping indicate that basement units define four types of occurrences, including (1) xenoliths of ca. 1.33 to ≥1.18 Ga age, (2) an early magmatic suite including meta-granitoids of ca. 1185–1140 Ma age that enclose or locally intrude the xenoliths, (3) metasedimentary rocks represented by layered granofels and biotite schist whose protoliths were likely deposited on the older meta-granitoids, and (4) a late magmatic suite composed of younger, ca. 1075–1030 Ma intrusive rocks of variable chemical composition that intruded the older rocks. The magmatic protolith of granofels constituting part of a layered, map-scale xenolith crystallized at ca. 1327 Ma, indicating that the lithology represents the oldest, intact crust presently recognized in the southern Appalachians. SHRIMP U-Pb data indicate that periods of regional Mesoproterozoic metamorphism occurred at 1170–1140 and 1070–1020 Ma. The near synchroneity in timing of regional metamorphism and magmatism suggests that magmas were emplaced into crust that was likely at near-solidus temperatures and that melts might have contributed to the regional heat budget. Much of the area is cut by numerous, generally east- to northeast-striking Paleozoic fault zones characterized by variable degrees of ductile deformation and recrystallization. These high-strain fault zones dismember the terrane, resulting in juxtaposition of units and transformation of basement lithologies to quartz- and mica-rich tectonites with protomylonitic and mylonitic textures. Mineral assemblages developed within such zones indicate that deformation and recrystallization likely occurred at greenschist-facies conditions at ca. 340 Ma.

North Carolina, Virginia↗
USGS2012

SHRIMP U-Pb ages of xenotime and monazite from the Spar Lake red bed-associated Cu-Ag deposit, western Montana: Implications for ore genesis

Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1–30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornite-calcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207 Pb/ 206 Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207 Pb/ 206 Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization.

Montana↗
USGS2012

Multiple age components in individual molybdenite grains

Re–Os geochronology of fractions composed of unsized, coarse, and fine molybdenite from a pod of unusual monazite–xenotime gneiss within a granulite facies paragneiss, Hudson Highlands, NY, yielded dates of 950.5 ± 2.5, 953.8 ± 2.6, and 941.2 ± 2.6 Ma, respectively. These dates are not recorded by co-existing zircon, monazite, or xenotime. SEM–BSE imagery of thin sections and separated grains reveals that most molybdenite grains are composed of core and rim plates that are approximately perpendicular. Rim material invaded cores, forming irregular contacts, probably reflecting dissolution/reprecipitation. EPMA and LA-ICP-MS analyses show that cores and rims have different trace element concentrations (for example, cores are relatively enriched in W). On the basis of inclusions of zircon with metamorphic overgrowths, we conclude that molybdenite cores and rims formed after high-grade regional metamorphism. The discovery of cores and rims in individual molybdenite grains is analogous to multi-component U-Pb geochronometers such as zircon, monazite, and titanite; thus, molybdenite should be carefully examined before dating to ensure that the requirement of age homogeneity is fulfilled.

New York↗
USGS2012

Neoproterozoic tectonic evolution of the Jebel Saghro and Bou Azzer - El Graara inliers, eastern and central Anti-Atlas, Morocco

New mapping, geochemistry, and 17 U–Pb SHRIMP zircon ages from rocks of the Sirwa, Bou Azzer–El Graara, and Jebel Saghro inliers constrain the Neoproterozoic evolution of the eastern Anti-Atlas during Pan-African orogenesis. In the Sirwa inlier, Tonian quartzite from the pre Pan-African passive margin deposits of the Mimount Formation contains detrital zircon derived entirely from the West African Craton (WAC), with most grains yielding Eburnean Paleoproterozoic ages of about 2050 Ma. Cryogenian Pan-African orogenic activity (PA1) from about 760 to 660 Ma included northward-dipping subduction to produce a volcanic arc, followed by ophiolite obduction onto the WAC. In the Bou Azzer–El Graara inlier, calc-alkaline granodiorite and quartz diorite, dated at 650–646 Ma, are syn- to post-tectonic with respect to the second period of Pan-African orogenesis (PA2), arc-continent accretion, and related greenschist facies metamorphism. Slab break-off and lithospheric delimination may have provided the source for the supra-subduction calc-alkaline plutons. At about 646 Ma, quartz diorite intruded the Tiddiline formation placing an upper limit on molassic deposition. Widespread Ediacaran high-K calc-alkaline to shoshonitic plutonism and volcanism during the final stage of Pan-African orogenesis (PA3) occurred in a setting related to either modification of the margin of the WAC or formation of a continental volcanic arc above a short-lived southward-dipping subduction zone. In the Saghro inlier, eight plutonic rocks yield ages ranging from about 588 to 556 Ma. Sampled plutonic rocks previously considered to be Cryogenian yielded Ediacaran ages. Peraluminous rhyolitic volcanic rocks in the lower part of the Ouarzazate Supergroup, including ash-flow tuffs of the Oued Dar’a caldera, yield ages between about 574 and 571 Ma. The Oued Dar’a caldera developed in a pull-apart graben produced by a left-step in a northeast-trending, left-lateral strike-slip fault zone, and much of the lower Ouarzazate Supergroup volcanic rocks in the area are probably related to caldera out-flow facies and collapse. Late stage PA3 intrusive rocks include the Bouskour–Sidi Flah and Timijt rhyolitic dike swarms at about 563 Ma, the voluminous pink Isk-n-Alla granite (559 ± 5 Ma), and volumetrically minor gabbro of Tagmout (556 ± 5 Ma). Rhyolite flows from the upper part of the Ouarzazate Supergroup, above a regional angular unconformity, yielded ages of 558 ± 4 and 556 ± 4 Ma. The youngest ages place an upper limit on block faulting and weak folding during latest Pan-African tectonic activity (PA3), coincident with the departure of the Cadomian crustal fragment from the northern margin of the WAC.

Precambrian Research↗
USGS2012

Synthesis of petrographic, geochemical, and isotopic data for the Boulder batholith, southwest Montana

The Late Cretaceous Boulder batholith in southwest Montana consists of the Butte Granite and a group of associated smaller intrusions emplaced into Mesoproterozoic to Mesozoic sedimentary rocks and into the Late Cretaceous Elkhorn Mountains Volcanics. The Boulder batholith is dominated by the voluminous Butte Granite, which is surrounded by as many as a dozen individually named, peripheral intrusions. These granodiorite, monzogranite, and minor syenogranite intrusions contain varying abundances of plagioclase, alkali feldspar, quartz, biotite, hornblende, rare clinopyroxene, and opaque oxide minerals. Mafic, intermediate, and felsic subsets of the Boulder batholith intrusions are defined principally on the basis of color index. Most Boulder batholith plutons have inequigranular to seriate textures although several are porphyritic and some are granophyric (and locally miarolitic). Most of these plutons are medium grained but several of the more felsic and granophyric intrusions are fine grained. Petrographic characteristics, especially relative abundances of constituent minerals, are distinctive and foster reasonably unambiguous identification of individual intrusions. Seventeen samples from plutons of the Boulder batholith were dated by SHRIMP ( S ensitive H igh R esolution I on M icroprobe) zircon U-Pb geochronology. Three samples of the Butte Granite show that this large pluton may be composite, having formed during two episodes of magmatism at about 76.7 ± 0.5 Ma (2 samples) and 74.7 ± 0.6 million years ago (Ma) (1 sample). However, petrographic and chemical data are inconsistent with the Butte Granite consisting of separate, compositionally distinct intrusions. Accordingly, solidification of magma represented by the Butte Granite appears to have spanned about 2 million year (m.y.). The remaining Boulder batholith plutons were emplaced during a 6-10 m.y. span (81.7 ± 1.4 Ma to 73.7 ± 0.6 Ma). The compositional characteristics of these plutons are similar to those of moderately differentiated subduction-related magmas. The plutons form relatively coherent, distinct but broadly overlapping major oxide composition clusters or linear arrays on geochemical variation diagrams. Rock compositions are subalkaline, magnesian, calc-alkalic to calcic, and metaluminous to weakly peraluminous. The Butte Granite intrusion is homogeneous with respect to major oxide abundances. Each of the plutons is also characterized by distinct trace element abundances although absolute trace element abundance variations are relatively minor. Limited Sr and Nd isotope data for whole-rock samples of the Boulder batholith are more radiogenic than those for plutonic rocks of western Idaho, eastern Oregon, the Salmon River suture, and most of the Big Belt Mountains. Initial strontium (Sr i ) values are low and epsilon neodymium (ε Nd ) values are comparable relative to those of other southwest Montana basement and Mesozoic intrusive rocks. Importantly, although the Boulder batholith hosts significant mineral deposits, including the world-class Butte Cu-Ag deposit, ore metal abundances in the Butte Granite, as well as in its peripheral plutons, are not elevated but are comparable to global average abundances in igneous rocks.

Montana↗
USGS2012

Geologic map of the Great Smoky Mountains National Park region, Tennessee and North Carolina

The geology of the Great Smoky Mountains National Park region of Tennessee and North Carolina was studied from 1993 to 2003 as part of a cooperative investigation by the U.S. Geological Survey with the National Park Service (NPS). This work resulted in a 1:100,000-scale geologic map derived from mapping that was conducted at scales of 1:24,000 and 1:62,500. The geologic data are intended to support cooperative investigations with the NPS, the development of a new soil map by the Natural Resources Conservation Service, and the All Taxa Biodiversity Inventory. In response to a request by the NPS, we mapped previously unstudied areas, revised the geology where problems existed, and developed a map database for use in interdisciplinary research, land management, and interpretive programs for park visitors.

North Carolina, Tennessee↗
USGS2011

The Edwardsburg Formation and related rocks, Windermere Supergroup, central Idaho, USA

In central Idaho, Neoproterozoic stratified rocks are engulfed by the Late Cretaceous Idaho batholith and by Eocene volcanic and plutonic rocks of the Challis event. Studied sections in the Gospel Peaks and Big Creek areas of west-central Idaho are in roof pendants of the Idaho batholith. A drill core section studied from near Challis, east-central Idaho, lies beneath the Challis Volcanic Group and is not exposed at the surface. Metamorphic and deformational overprinting, as well as widespread dismembering by the younger igneous rocks, conceals many primary details. Despite this, these rocks provide important links for regional correlations and have produced critical geochronological data for two Neoproterozoic glacial periods in the North American Cordillera. At the base of the section, the more than 700-m-thick Edwardsburg Formation (Fm.) contains interlayered diamictite and volcanic rocks. There are two diamictite-bearing members in the Edwardsburg Fm. that are closely related in time. Each of the diamictites is associated with intermediate composition tuff or flow rocks and the diamictites are separated by mafic volcanic rocks. SHRIMP U–Pb dating indicates that the lower diamictite is about 685±7 Ma, whereas the upper diamictite is 684±4 Ma. The diamictite units are part of a cycle of rocks from coarse clastic, to fine clastic, to carbonate rocks that, by correlation to better preserved sections, are thought to record an older Cryogenian glacial to interglacial period in the northern US Cordillera. The more than 75-m-thick diamictite of Daugherty Gulch is dated at 664±6 Ma. This unit is preserved only in drill core and the palaeoenvironmental interpretation and local stratigraphic relations are non-unique. Thus, the date for this diamictite may provide a date for a newly recognized glaciogenic horizon or may be a minimum age for the diamictite in the Edwardsburg Fm. The c. 1000-m-thick Moores Lake Fm. is an amphibolite facies diamictite in which glacial features have not been observed. However, it is part of a sedimentary cycle from unsorted siliclastic deposits to mud and carbonate deposits. Using lithostratigraphy and available geochronology, the Moores Lake Fm. is correlated with a younger succession of Cryogenian glaciogenic rocks in southeastern Idaho. Traditional correlations of Neoproterozoic rocks in the Cordillera recognize two levels of Cryogenian diamictites. The Edwardsburg and Moores Lake diamictites along the middle Cordillera fit well into the scenario of two glacial events. Because of the correlations, dates that provide ages for the diamictites in central Idaho (and corroborated in southeastern Idaho, Link & Fanning 2008) could constrain the age of correlated glaciogenic deposits elsewhere in the Cordillera. However, in the absence of dates for the glaciogenic diamictites in Canadian and southern US Cordilleran sections, the correlations are considered possible but uncertain.

Idaho↗
USGS2011

Provisional zircon and monazite uranium-lead geochronology for selected rocks from Vermont

This report presents the results of zircon and monazite uranium-lead (U-Pb) geochronologic analyses of 24 rock samples. The samples in this study were collected from mapped exposures identified while conducting either new, detailed (1:24,000-scale) geologic quadrangle mapping or reconnaissance mapping, both of which were used for compilation of the bedrock geologic map of Vermont. All of the collected samples were judged to be igneous rocks (either intrusive or extrusive) on the basis of field relations and geochemistry. The one exception is the Okemo Quartzite on Ludlow Mountain. These geochronologic data were used to supplement regional correlations between igneous suites on the basis of similar geochemistry and geologic mapping.

Open-File Report↗
USGS2011

Bedrock geologic map of the Grafton quadrangle, Worcester County, Massachusetts

The bedrock geology of the 7.5-minute Grafton, Massachusetts, quadrangle consists of deformed Neoproterozoic to early Paleozoic crystalline metamorphic and intrusive igneous rocks. Neoproterozoic intrusive, metasedimentary, and metavolcanic rocks crop out in the Avalon zone, and Cambrian to Silurian intrusive, metasedimentary, and metavolcanic rocks crop out in the Nashoba zone. Rocks of the Avalon and Nashoba zones, or terranes, are separated by the Bloody Bluff fault. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing hydrogeologic characterization of the fractured bedrock of Massachusetts. This report presents mapping by G.J. Walsh, geochronology by J.N. Aleinikoff, geochemistry by M.J. Dorais, and consists of a map, text pamphlet, and GIS database. The map and text pamphlet are available in paper format or as downloadable files (see frame at right). The GIS database is available for download. The database includes contacts of bedrock geologic units, faults, outcrops, structural geologic information, and photographs.

Massachusetts↗
USGS2010

Contrasting P‐T‐t paths: Thermochronologic evidence for a Late Paleozoic final assembly of the Avalon Composite Terrane in the New England Appalachians

Strongly contrasting pressure‐temperature‐time paths for the Avalon composite terrane and the structurally overlying Putnam‐Nashoba zone in eastern New England obtained from thermochronologic and thermobarometric data are best explained by a late Paleozoic underthrusting of cover rocks by the Avalon composite terrane. We present new Ar and U‐Pb thermochronologic data that show that in the southern Hope Valley zone, Permian (280 Ma) anatectic metamorphic conditions of 700°C and 6 kbar were quenched by relatively rapid cooling (12°C/m.y.) and exhumation (0.5 km/m.y.) for ∼40 m.y. In contrast, peak metamorphic conditions in the Putnam‐Nashoba zone predate Silurian intrusions, and slower cooling (3.5°C/m.y.) began at about 400 Ma. One‐dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the Permian metamorphism of the Avalon composite terrane. Because of the absence of high‐grade Alleghanian metamorphism in rocks overlying the Avalon terrane, we conclude that high‐grade Alleghanian metamorphism in the Avalon terrane occurred east of rocks now overlying it and that significant motion between Avalon and this cover occurred after peak Alleghanian metamorphism. Similarly contrasting metamorphic histories between Avalon inliers (Willimantic window, Massabesic complex gneiss, Pelham dome) and their cover rocks reveals the regional significance of this boundary. The core rocks all show Permian cooling, but the cover rocks show post‐Acadian cooling ages decreasing from east to west to the Pelham area, where hornblende ages in Avalon and cover differ by only 35 rather than 80 m.y. Model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years. Consequently, underthrusting of Avalon is constrained to be middle Mississippian or younger. Because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover, core rocks of the Pelham dome must have been relatively close to this leading edge. Thus Carboniferous to Permian underplating from a generally eastward direction best explains these thermochronologic relationships.

Connecticut, Maine, Massachusetts, New Hampshire, ↗