Deep crustal xenoliths from central Montana, USA: Implications for the timing and mechanisms of high-velocity lower crust formation

被引:25
作者
Barnhart, Katherine R. [1 ]
Mahan, Kevin H. [1 ]
Blackburn, Terrence J. [2 ]
Bowring, Samuel A. [2 ]
Dudas, Francis O. [2 ]
机构
[1] Univ Colorado, Dept Geol Sci, Boulder, CO 80309 USA
[2] MIT, Dept Earth Atmospher & Planetary Sci, Cambridge, MA 02139 USA
基金
美国国家科学基金会;
关键词
LOWER CONTINENTAL-CRUST; FALLS TECTONIC ZONE; SOUTHEASTERN BRITISH-COLUMBIA; CENTRAL WYOMING PROVINCE; UPPER-MANTLE BENEATH; RB-SR GEOCHRONOLOGY; U-PB GEOCHRONOLOGY; EAST-CENTRAL IDAHO; UNITED-STATES; BELT BASIN;
D O I
10.1130/GES00765.1
中图分类号
P [天文学、地球科学];
学科分类号
07 ;
摘要
Integration of petrologic, chronologic and petrophysical xenolith data with geophysical observations can offer fundamental insights into understanding the evolution of continental crust. We present the results of a deep crustal xenolith study from the northern Rocky Mountain region of the western U. S., where seismic experiments reveal an anomalously thick (10-30 km), high seismic velocity (compressional body wave, Vp > 7.0 km/s) lower crustal layer, herein referred to as the 7.x layer. Xenoliths exhumed by Eocene minettes from the Bearpaw Mountains of central Montana, within the Great Falls tectonic zone, include mafic and intermediate garnet granulites, mafic hornblende eclogite, and felsic granulites. Calculated pressures of 0.6-1.5 GPa are consistent with derivation from 23-54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories including prograde burial and episodes of decompression. Samples with barometrically determined depths consistent with residence within the seismically defined 7.x layer have calculated bulk P-wave velocities of 6.9-7.8 km/s, indicating heterogeneity in the layer. Shallower samples have markedly slower velocities consistent with seismic models. New monazite total U-Th-Pb data and a variety of additional published geochronology indicate a prolonged and episodic metamorphic history, beginning with protolith ages as old as Archean and followed by metamorphic and deep crustal fluid-flow events ca. 2.1 Ga, 1.8-1.7 Ga, and 1.5-1.3 Ga. We suggest that the 7.x layer in this region owes its character to a variety of processes, including magmatic underplating and intraplating, associated with multiple tectonic events from the Neo-archean to the Mesoproterozoic.
引用
收藏
页码:1408 / 1428
页数:21
相关论文
共 123 条
[1]  
Alcock J., 2010, GEOLOGICAL SOC AM AB, V42, P415
[2]   U-PB GEOCHRONOLOGY OF THE MOYIE SILLS, PURCELL SUPERGROUP, SOUTHEASTERN BRITISH-COLUMBIA - IMPLICATIONS FOR THE MESOPROTEROZOIC GEOLOGICAL HISTORY OF THE PURCELL (BELT) BASIN [J].
ANDERSON, HE ;
DAVIS, DW .
CANADIAN JOURNAL OF EARTH SCIENCES, 1995, 32 (08) :1180-1193
[3]   A 3-D shear velocity model of the crust and uppermost mantle beneath the United States from ambient seismic noise [J].
Bensen, G. D. ;
Ritzwoller, M. H. ;
Yang, Y. .
GEOPHYSICAL JOURNAL INTERNATIONAL, 2009, 177 (03) :1177-1196
[4]   Reversed phase equilibrium constraints on the stability of Mg-Fe-Al biotite [J].
Berman, R. G. ;
Aranovich, L. Ya. ;
Rancourt, D. G. ;
Mercier, P. H. J. .
AMERICAN MINERALOGIST, 2007, 92 (01) :139-150
[5]   Optimized standard state and solution properties of minerals .1. Model calibration for olivine, orthpyroxene, cordierite, garnet, ilmenite in the system FEO-MGO-CaO-Al2O3-TiO3-SiO2 [J].
Berman, RG ;
Aranovich, LY .
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1996, 126 (1-2) :1-24
[6]  
BERMAN RG, 1991, CAN MINERAL, V29, P833
[7]  
BICKFORD ME, 1990, GEOLOGY, V18, P3
[8]  
Blackburn T.J., 2010, EARTHSCOPE NEWSL SPR
[9]   Zirconium in rutile speedometry: New constraints on lower crustal cooling rates and residence temperatures [J].
Blackburn, Terrence ;
Shimizu, Nobumichi ;
Bowring, Samuel A. ;
Schoene, Blair ;
Mahan, Kevin H. .
EARTH AND PLANETARY SCIENCE LETTERS, 2012, 317 :231-240
[10]   U-Pb thermochronology: creating a temporal record of lithosphere thermal evolution [J].
Blackburn, Terrence ;
Bowring, Samuel A. ;
Schoene, Blair ;
Mahan, Kevin ;
Dudas, Francis .
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 2011, 162 (03) :479-500