ABSTRACT Acadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low‐ P , high‐ T (Buchan‐type) metamorphism followed by thickening at high temperature (>650d̀ C) and then by cooling to 100‐200d̀ C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low‐ P cordierite‐bearing assemblages by high‐ P garnet‐bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower‐ T and higher‐ P assemblages. Peak conditions in the highest grade rocks were 685‐780d̀ C and 5‐6 kbar; the cooling path passed through 550d̀ C at about 6.5 kbar. The well‐constrained P‐T path documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long‐lived metamorphic event spanning a broad range of P‐T conditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO 2 ‐rich, mixed CO 2 ‐N 2 ‐rich, N 2 ‐rich and H 2 O‐rich) and densities during metamorphism. Densities of CO 2 fluid inclusions range from 0.20 to 1.03 g cm ‐3 . Medium‐density CO 2 fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650‐700d̀ C at pressures below 5 kbar. Other CO 2 fluid inclusions from matrix quartz yield isochores which pass through 700d̀ C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high‐ P assemblages have isochores which pass below the estimated P‐T conditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low‐ P heating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500d̀ C with later decompression. This interpretation is also consistent with previously published petrological models and supports an anticlockwise P‐T path for the CMT of south‐central Massachusetts.
Research Article| December 01, 1994 Direct evidence for a steep geotherm under conditions of rapid denudation, Western Himalaya, Pakistan David M. Winslow; David M. Winslow 1Department of Earth and Environmental Sciences, 31 Williams Drive, Lehigh University, Bethlehem, Pennsylvania 18015 Search for other works by this author on: GSW Google Scholar Peter K. Zeitler; Peter K. Zeitler 1Department of Earth and Environmental Sciences, 31 Williams Drive, Lehigh University, Bethlehem, Pennsylvania 18015 Search for other works by this author on: GSW Google Scholar C. Page Chamberlain; C. Page Chamberlain 2Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Lincoln S. Hollister Lincoln S. Hollister 3Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (12): 1075–1078. https://doi.org/10.1130/0091-7613(1994)022<1075:DEFASG>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David M. Winslow, Peter K. Zeitler, C. Page Chamberlain, Lincoln S. Hollister; Direct evidence for a steep geotherm under conditions of rapid denudation, Western Himalaya, Pakistan. Geology 1994;; 22 (12): 1075–1078. doi: https://doi.org/10.1130/0091-7613(1994)022<1075:DEFASG>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Recent fluid-inclusion and 40Ar/39Ar cooling-age data show that currently exposed basement rocks in the Raikhot glacier valley of the Nanga Parbat-Haramosh massif, Pakistan Himalaya, were at temperatures of 350 ± 50 °C at depths of 6 ± 2 km (hydrostatic pressure correction). These data imply the presence of a steep thermal gradient in the upper crust at 1 Ma (29-100 °C/km) and denudation rates over the past 1.0 m.y. of 3-6 mm/yr, providing independent corroboration of previous estimates of rapid denudation at Nanga Parbat (4.5 mm/yr over 3.3 m.y.). Our data provide direct documentation of near-surface compaction of isotherms under conditions of rapid denudation, a result that has long been supported by thermal modeling. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The Nanga Parbat-Haramosh Massif of the western Himalaya has undergone a complex metamorphic and denudational history over the past 1.8 Ga. Metamorphism in the Tato region is characterized by a Barrovian type metamorphism followed by partial melting and recrystallization along shear zones during nearly isothermal decompression while the massif was still at temperatures >550°C. Mineralogical and textural evidence for this path includes garnet zoning patterns, late-stage migmatization, abundant cordierite both in schists and leucogranite dikes, replacement of high-pressure assemblages by low-pressure assemblages, and abundant low-density inclusions. Thermobarometry on the nonmigmatized gneisses reveals conditions of 540-740°C at 7.1-13.1 kbar. Thermobarometry in the migmat-ized rocks reveals final equilibration at 608-675°C at 3.9-6.8 kbar. Early fluid inclusions occur in quartz inclusions within garnet porphyroblasts that grew during decompression. The last fluid inclusions to be trapped occur in microfractures that locally crosscut several grain boundaries and, hence, record conditions after quartz grain boundary migration ceased. Examination of all the petrologic and fluid inclusions data conclusively show a very rapid denudation event late in the collisional process. Rapid denudation resulted in the advection of isotherms to shallow crustal levels causing an elevated geotherm (~60°C/km) in the upper crust, metamorphism along shear zones, and partial melting of the crust at depths of ca. 22 km.
40 Ar/ 39 Ar data (hornblende, biotite, muscovite, and K‐feldspar) and U/Pb data (zircons) were obtained from the Nanga Parbat‐Haramosh Massif (NPHM), NW Pakistan, along three transects in the southern regions of the NPHM. We have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif. Geochronologic data show that the NPHM has experienced exceptionally high denudation and cooling rates over the past 10 m.y. U/Pb ages determined through sensitive high‐resolution ion microprobe (SHRIMP) “depth‐profiling” experiments on metamorphic zircons and conventional U/Pb monazite dates suggest that the timing of metamorphism varied across the massif. In addition, we have documented that the massif has experienced postmetamorphic, differential cooling both along and across strike. Thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south‐central regions of the massif (representing deeper crustal levels) than along the margins and northern regions of the massif. Within the Tato region, cooling following peak metamorphic temperatures of 600°–700 °C was as high as 140 °C/m.y. following partial melting of pelitic units. Biotites from this area record plateau ages of 0.9 ± 0.1 Ma. Along the Astor and Indus gorges, cooling was less rapid (approximately 70°–80°C/m.y.) following peak metamorphism as indicated by U/Pb monazite ages of 6–8 Ma and 40 Ar/ 39 Ar muscovite cooling ages of 2.2–3.4 Ma. Cooling over the last 3 m.y. occurred at rates of 100°–140 °C/m.y. The overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 Ma, followed by reverse faulting along east dipping fault zones. Along the Raikot River transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces. The age pattern was disrupted at 1 Ma due to faulting along the Raikot and Tato faults. An electronic supplement of Tables A1, A2, and A3 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GUEST and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC00032, Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan, David M. Winslow, Peter K. Zeitler, C. Page Chamberlain, and Ian S. Williams). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $$15.00. Payment must accompany order.
During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 Ma), relatively low δ 18 O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 Ar/ 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
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