Eo-Variscan Orogenesis in the Guilleries Massif, Catalan Coastal Ranges, Northeastern
Total Page：16
File Type：pdf, Size：1020Kb
Eo-Variscan orogenesis in the Guilleries Massif, Catalan Coastal Ranges, Northeastern Spain recorded by U-Th-Pb ages of monazite inclusions in metamorphic garnet A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science In the Department of Geology of the College of Arts and Sciences By Julia Wise B.A. Macalester College, 2007 May 2012 Committee Chair: Craig Dietsch, Ph.D. 1 Abstract: A chronicle of the Variscan Orogeny is recorded in high grade pelitic schists of the Guilleries Massif of the Catalan Coastal Ranges of northeastern Spain which preserve multiple rounds of deformation and metamorphism. The massif records a classic Variscan low-pressure and high-temperature metamorphic field gradient and represents part of the metamorphic core of the Variscan Orogeny. The lack of overprinting from the younger Alpine Orogeny makes the massif an ideal location to study the early stages of the Variscan Orogeny. Pelitic schist and gneiss of the Osor formation are characterized by andalusite + cordierite and biotite + garnet + sillimanite assemblages. In garnet porphyroblasts, folded S1 inclusion trails with monazite are truncated by the regionally dominant S2 cleavage. Andalusite + cordierite grade rocks and S2 are syntectonic with the 323 Ma Susqueda Diorite. In the aureole of the Susqueda Diorite, contact metamorphism reached pyroxene + garnet + cordierite grade. Biotite and two-mica microgranites, ranging from meter-sized dikes and sills to mm-sized veins, trending northeast with crystallization ages of ca. 300 Ma, cross-cut all the country rocks of the 232 208 massif. Th- Pb ages of monazite from preserved S1 inclusion trails are 341 Ma, 340 Ma, and 232 208 334 Ma and Th- Pb ages of monazite from S2 inclusion trails are 312 Ma and 313 Ma. The older ages record a phase of deformation and metamorphism that predates the peak low pressure- high temperature Variscan thermal metamorphism that is related to the intrusion of granite. Relict kyanite preserved in the matrix of rocks with S1 inclusion trails provides evidence for a phase of nonmagmatic thrusting and higher pressure metamorphism prior to the peak metamorphic event. 2 Table of Contents Chapter 1: Introduction and Methods--------pg. 5 Chapter 2: Lithologies and Garnet Types----pg. 21 Chapter 3: Th-Pb Ages of Monazite----------pg. 28 Chapter 4: Discussion---------------------------pg. 31 Figures and Tables------------------------------pg. 35 References-----------------------------------------pg. 47 3 List of Tables and Figures Figure 1: Reconstruction of Rheic Ocean, ca. 480 Ma.----------------------------pg. 36 Figure 2: Location of Variscan Massifs in Modern Western Europe-------------pg. 37 Figure 3: Modern Global Variscan Belts---------------------------------------------pg. 38 Figure 4: Catalan Coastal Ranges of Northeastern Spain----------------------------pg.39 Figure 5: Geologic Map of the Guilleries Massif with Metamorphic Grades---pg. 40 Figure 6: Locations of Samples Collected--------------------------------------------pg. 41 Figure 7: Garnet Type A----------------------------------------------------------------pg. 42 Figure 8: Garnet Type B----------------------------------------------------------------pg. 43 Figure 9: Garnet Type C----------------------------------------------------------------pg. 44 Figure 10: Summary of U-Th-Pb Series Dating-------------------------------------pg. 45 Table 1: Reduced Raw Data from U-Th-Pb Series Dating—----------------------pg. 46 4 Chapter 1: Introduction and Methods 5 Introduction: The geography of Earth is not fixed: the continents are constantly in motion. They rift apart along lines of weakness and, in turn, collide, creating new topographies of volcanoes, valleys, and mountain ranges. The creation of a mountain range is known as an orogeny and just as there is variety in the landscapes on the Earth there are a variety of ways orogenies occur. For example, mountains can be built by the subduction of one tectonic plate under another; this is the tectonic process that that created the Andes. Orogeny can also occur through a collision of two or more large continents, such as the collision between India and Asia that is currently uplifting the Himalayas. My research considers one continental-continental collisional orogeny: the Late Paleozoic Variscan Orogeny. The Variscan Orogeny was the result of a collision between the paleocontinents Gondwana and Laurussia. This collision was responsible for the closure of the Rheic Ocean and assembly of the supercontinent Pangaea (Murphy and Nance, 2004) which was completed during the Permian Period. Some of the rocks created during the Variscan Orogeny reflect the inland processes of a mountain building event. These are the rocks of the extremities of the Variscan belt, and include the Guilleries massif of the Catalonian Coastal Ranges of northeastern Spain that were scarred with deformation early on during the collision of Gondwana and Laurussia, from stresses traveling through weakened crust. This study addresses the evidence of the early stages of the Variscan Orogeny — the Eo-Variscan —as preserved in the Guilleries massif. It takes into consideration the collision of the western portion of Gondwana with southern Baltica, Understanding Continental—Continental Collisions: Continental-continental collisions begin with the activation of a subduction zone as two continents are drawn towards each other. 6 Eventually, the oceanic crust separating the two continents is consumed and the two slabs of continental crust begin to collide. This crustal collision results in the creation of fold and thrust belts, the stacking of nappes, crustal thickening, and the overall elevational uplift of a new mountain range. As the crust accumulates and thickens, a metamorphic core is created. Powered by the heat transfer in the accumulating, thickening crust, and heat input from the underlying mantle, this core provides the energy for metamorphism, crustal melting, and the uplift of high mountains. Upon the end of the collision, the crust is at its thickest and heating continues. As the increasing heat produces partial melting of deeply buried rocks, the crust softens. This softening coupled with the lessening of the compressional regime of collision, can lead to extensional collapse, decompressional melting, and the production of large granitic intrusions (Kearey et al., 2009). An orogeny is not a smooth flowing process or even a head-on collision like a train wreck. Rather, orogenies occur in fits and starts of collision, deformation, and metamorphism. Most collisional zones preserve slices of continental crust that have undergone multiple phases of reworking. During younger phases of deformation and metamorphism, the older history of collisions imprinted on rocks through their mineral assemblages and deformational fabrics is mostly erased. Still, traces of these early stages may be preserved and can be seen, for example, in low-strain domains as unique textures, conglomerates with deformed granite clasts, relict mineral assemblages recording prior grades of metamorphism, and as mineral inclusions preserved in relic mineral phases, even in highly altered domains—such as inclusion trails in garnet. The precollisional history of the lithospheric plates involved in continental-continental collisional orogenies influences the collisional path that builds the resultant mountain range. The 7 lithospheric crust of the paleocontinents Gondwana and Laurussia involved in the Variscan Orogeny was weakened prior to the main stage of the orogeny by the repeated rifting and accretion of multiple small terranes, from and to their edges (Stampfli et al., 2002). These repeated stresses, coupled with the collisional stress of the Neoproterozoic Cadomian Orogeny during the pre-Variscan closure of the Iapatus Ocean, weakened the Laurussian and Gondwanan lithospheric plates. Loss of crustal integrity was caused by the repeat faulting, breaking, and suturing along the margins of Gondwana and Laurussia, so that deformation from the Variscan collision could travel far inboard. In this manner, travelling along weak surfaces in the crust such as faults, deformation traveled away from the immediate collision zone and into the surrounding uplifting areas of the growing mountain range (Stampfli et al., 2002) This “preconditioning” of the crust may have been responsible for the appearance of the earliest deformation of the Variscan orogeny seen in the Guilleries massif. Matte (1986) hypothesized that already weak crust was responsible for increased indentation of Laurussia by Gondwana as the crust of the former continent folded and faulted to accommodate the latter’s collision with it. Essentially, the pre-Variscan tectonic history of crust involved in the Variscan collision could explain why far-field effects of deformation accompanying the Variscan continental collision are so widespread (Echtler and Malavieille, 1989). The large geographic expanse of this orogeny, throughout which different stages of the collision are preserved, makes the Variscan an ideal model for understanding the complicated history of a continental- continental collision. 8 Plate Tectonic Scenario of the Variscan Orogeny: The Rheic Ocean was bordered to the north by Laurussia and to the south by Gondwana. Gondwana was an expansive continent composed of modern day Africa, India, Antarctica, Australia, and South America. While at its greatest extent, Laurussia was