B6: Devonian Granite Melt Transfer in Western Maine: Relations Between Deformation, Metamorphism, Melting and Pluton Emplacement at the Migmatite Front

Guidebook for field trips in Western Maine and Northern New Hampshire: New England Intercollegiate Geological Conference, p. 217-246

The Migmatite-Granite Complex of southern Maine: Its structure, petrology, geochemistry, geochronology, and relation to the Sebago Pluton

in Berry, Henry N., IV, and West, David P., Jr., editors, Guidebook for field trips along the Maine coast from Maquoit Bay to Muscongus Bay: New England Intercollegiate Geological Conference, p. 19-42https://digitalmaine.com/mgs_publications/1020/thumbnail.jp

The Migmatite-Granite Complex of southern Maine: Its structure, petrology, geochemistry, geochronology, and relation to the Sebago Pluton

in Berry, Henry N., IV, and West, David P., Jr., editors, Guidebook for field trips along the Maine coast from Maquoit Bay to Muscongus Bay: New England Intercollegiate Geological Conference, p. 19-42https://digitalmaine.com/mgs_publications/1020/thumbnail.jp

Centimeter- To Meter-Scale Structural and Petrographic Variations in a 3-D Exposure in the Migmatite Granite Complex, Southern Maine

Migmatites are a meta-igneous rock which crystallized early in the stages of melt, capturing the process of anatexas, or partial melting. As part of a multi-disciplinary study of rocks in the Migmatite-Granite belt of the Northern Appalachians, we have documented variations at the cm- to m- scale of fabrics and geometry of granitic bodies in a large, 3-D exposure in W. Cumberland, ME. This exposure is new as the homeowners have removed glacial till from the property, re-exposing glacially polished 76m x 24m pavement adjacent to a 15m high cliff face. An exposure of this magnitude and detail is rare since it is in an area of low outcrop density. This provided a unique opportunity to study a large 3-D outcrop within the Migmatite-Granite Complex (MGC). The outcrop is dominated by metasedimentary metatexite migmatite which includes two 3-m-wide concordant amphibolite layers. The fabric of the outcrop exhibits a NE-SW trend with moderate SE dips. This outcrop resembles the same fabric found in regional data, which was expected. Dextral shear is indicated by asymmetric trails around porphyroblasts, boudinage, and ptygmatic folds of stromatic migmatite. The fabric indicating the shear is found within leucosomes, granitic bodies within the migmatite, indicating younger deformation than migmatite formation; this is also consistent with regional MGC data. Migmatite layers are intruded by granitic rocks with grain sizes from coarse to pegmatitic. Observations of contacts indicate cm-scale bake zones, also indicating the granite intruded following the formation of migmatite. Granite bodies are generally cm- to m- thick 2-mica coarse-grained granite to pegmatite. Cm-scale, medium grained granitic dikes are located throughout the outcrup cutting all structures, consistent with other rocks found throughout the MGC

The control of Lithium Budgets in Island Arcs

Measurements of the Li isotopic compositions of lavas from magmatic arcs worldwide suggest common processes at work that lead to the retention of isotopically heavy Li in the mantle. Samples from this study derive from the Kurile arc, eastern Russia, the Sunda arc, Indonesia, and a segment of the Aleutian arc, western Alaska. The overall range in δ7Li is very restricted (+2.1 to+5.1±1.1, 2σ) for 34 of 36 samples. These values overlap the values of unaltered normal MORB glasses. The two samples with isotopic compositions that fall outside this range in δ7Li have B/Be \u3c13, and hence do not bear classical ‘slab’ trace element signatures. Considering the high δ7Li in altered ocean crust, marine and terrigenous sediments, and forearc fluids, aqueous components lost by subducting slabs are expected to have similarly heavy enriched Li isotope signatures. If Li behaves similarly to a fluid-mobile element such as B, δ7Li should correlate strongly with, for example, B/Be. As such, samples with high B/Be should show elevated δ7Li. The sample set we have examined does not show such correlations and is interpreted to reflect a globally significant process. Although Li is a fluid-mobile element, its partitioning into Mg-silicates may cause it to be effectively removed during equilibration with subarc mantle peridotite. Elements with stronger fluid/mantle partitioning behavior, such as B, are not so affected. The convergence of Li isotope ratios on MORB-like values is interpreted to result from the sequestration of slab-derived Li in the subarc mantle before it reaches the zone of melting. The results indicate conditions appropriate for mantle ‘buffering’ of slab-derived Li are widespread in magmatic arcs. Alternately, some proportion of Li could be retained on the slab in high Li/B minerals. Either way, this indicates that regions of the upper mantle with δ7Li\u3eMORB may be common, as a direct consequence of the subduction process

The Boron Budget in Waters of the Mono Basin, California

Mono Lake in eastern California has the highest natural boron concentrations measured in a natural water body. Inputs to Mono Lake are from creeks that drain from the Sierra Nevada, accounting for over 80% of the total water input, and springs account for most of the rest of the water budget. We measured boron concentrations and isotope compositions of water sources in the lake and lake water collected over several seasons. The δ11B offset of at least +2.5‰ between Mono Lake water compared to its inputs suggests that, like seawater, the boron isotopic composition of the lake is influenced by the removal of light boron by coprecipitation and/or sorption of borate. Given the alkalinity of the lake, boron fractionation likely occurs before or as the water sources enter the lake. The famous tufa towers around the lake are a physical representation of a ‘chemical delta’ that alters the boron isotopic composition of the source fluids as they enter the lake. Based on different combinations of the measured end members, the residence time of boron in Mono Lake is estimated to be within the range of 5~80 ka

Birth, life, and demise of the Andean-syn-collisional Gissar arc: Late Paleozoic tectono-magmatic-metamorphic evolution of the southwestern Tian Shan, Tajikistan

The amalgamation of the Central Asian Orogenic Belt in the southwestern Tian Shan in Tajikistan is represented by tectono-magmatic-metamorphic processes that accompanied late Paleozoic ocean closure and collision between the Karakum-Tarim and Kazakh-Kyrgyz terranes. Integrated U-Pb geochronology, thermobarometry, pseudosection modeling, and Hf geochemistry constrain the timing and petro-tectonic nature of these processes. The Gissar batholith and the Garm massif represent an eastward, along-strike increase in paleodepth from upper-batholith (similar to 21-7km) to arc-root (similar to 36-19km) levels of the Andean-syn-collisional Gissar arc, which developed from similar to 323-288Ma in two stages: (i) Andean, I-type granitoid magmatism from similar to 323-306Ma due to northward subduction of the Gissar back-arc ocean basin under the Gissar microcontinent, which was immediately followed by (ii) syn-collisional, I-S-type granitoid magmatism in the Gissar batholith and the Garm massif from similar to 304-288Ma due to northward subduction/underthrusting of Karakum marginal-continental crust under the Gissar microcontinent. A rapid isotopic pull-up from similar to 288-286Ma signals the onset of juvenile, alkaline-syenitic, post-collisional magmatism by similar to 280Ma, which was driven by delamination of the Gissar arclogite root and consequent convective asthenospheric upwelling. Whereas M-HT/LP prograde metamorphism in the Garm massif (650-750 degrees C/6-7kbar) from similar to 310-288Ma was associated with subduction-magma inundation and crustal thickening, HT/LP heating and decompression to peak-metamorphic temperatures (similar to 800-820 degrees C/6-4kbar) at similar to 2886Ma was driven by the transmission of a post-collisional, mantle-derived heat wave through the Garm-massif crust.NSF Tectonics Program [NSF-EAR-1419748]; ExxonMobil; Geological Society of America; American Association of Petroleum Geologists; Coney fund; Conoco Philips scholarship; Arizona Laserchron Center [NSF-EAR-1338583]6 month embargo; Published online: 5 OCT 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]