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Home , Alpine orogeny, Cordierite, Metamorphic zone, Pelite

Eo-Variscan orogenesis in the Guilleries , Catalan Coastal Ranges, Northeastern

Spain recorded by U-Th-Pb ages of monazite inclusions in metamorphic

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 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 is recorded in high grade pelitic of the

Guilleries Massif of the Catalan Coastal Ranges of northeastern Spain which preserve multiple rounds of deformation and . 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 and of the Osor formation are characterized by andalusite + and + garnet + assemblages. In garnet porphyroblasts, folded S1 inclusion trails with monazite are truncated by the regionally dominant S2 . Andalusite + cordierite grade rocks and S2 are syntectonic with the 323 Ma Susqueda Diorite. In the aureole of the Susqueda Diorite, contact metamorphism reached + garnet + cordierite grade.

Biotite and two- 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 .

Relict 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.

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Table of Contents

Chapter 1: Introduction and Methods------pg. 5

Chapter 2: 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

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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

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Chapter 1: Introduction and Methods

5

Introduction:

The geography of Earth is not fixed: the continents are constantly in motion. They apart along lines of weakness and, in turn, collide, creating new topographies of volcanoes, valleys, and mountain ranges. The creation of a is known as an orogeny and just as there is variety in the landscapes on the Earth there are a variety of ways occur. For example, mountains can be built by the 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 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 and Laurussia. This collision was responsible for the closure of the

Rheic Ocean and assembly of the supercontinent (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 ,

Understanding Continental—Continental Collisions: Continental-continental collisions begin with the activation of a subduction zone as two continents are drawn towards each other.

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Eventually, the separating the two continents is consumed and the two slabs of continental crust begin to collide. This crustal collision results in the creation of 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 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 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 , from and to their edges (Stampfli et al., 2002). These repeated stresses, coupled with the collisional stress of the Neoproterozoic 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 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.

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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 , India, Antarctica,

Australia, and South America. While at its greatest extent, Laurussia was composed of modern day North America, Europe, and Asia. (Fig 1) At its broadest, the Rheic Ocean spanned nearly

4,000 km (Nance, 2008). The closure of the Rheic Ocean through the Variscan Orogeny in the northeast and the Ouachita-Alleghenian Orogeny in the southwest sutured together the paleocontinents Laurussia and Gondwana creating the supercontinent Pangaea, which represented the coalescence of all the land on the planet.

The Rheic Ocean began to shrink 480 million years ago with the activation of a subduction zone beneath the southern margin of Baltica. Following this was the appearance of arc magmatism in the Avalonian as the subduction transformed into a ridge and trench collision (Nance 2008). The Rheic Ocean continued to close from north to south in a zipper-like fashion with the majority of this intracontinental collision occurring during the Mississippian epoch (360-325 Ma) of the Carboniferous Period (360-299 Ma). The Rheic Ocean was completely consumed and Pangaea fully formed by 280 Ma. Closing of the Rheic Ocean created the bedrock of Western Europe and Eastern North America, raising mountain ranges from the

Ouachita Mountains to the Urals, and serving to produce the last, culminating phase of building the Appalachian Mountains.

The Variscan Orogeny was a global event. It created a contiguous mountain chain that ran from the Urals to the Ouachitas. Today the Variscan fingerprint is visible across

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Western Europe in a series of isolated large massifs characterized by andalusite-, sillimanite-, and higher-grade regional metamorphism (Fig 2 and Fig 3).

General Characteristics of the Variscan Orogeny and the Catalan Coastal Ranges:

Mineralogical, petrological, and geochronological evidence suggests that the thermally hottest Variscan metamorphism and most intense Variscan deformation from the collision occurred in a concentrated span of 40 million years from about 320 Ma-280 Ma (Martinez et al.

2008). This is known as peak Variscan metamorphism and deformation. However, there is evidence of an Eo-Variscan or pre-peak phase of deformation and metamorphism ca. 330-350

Ma.

The Variscan Orogeny produced the mineralogical hallmarks of low- pressure/high- temperature regional metamorphism. Sillimanite- to cordierite-K-feldspar-grade metamorphism associated with the wide-spread intrusion of S-type Carboniferous (ca 300-320 Ma) represent the maximum temperature conditions (Reche et al., 1998). S-type granites are formed from a sedimentary protolith. Variscan deformation and metamorphism affected the interior continental crust of Laurussia and Gondwana, and the final stages of deformation and metamorphism were also recorded away from magmatic arcs and subduction zones.

In review, the Variscan Orogeny had its origins as a subduction zone in the Rheic Ocean and morphed into a ridge-trench collision where a spreading ridge was consumed by a growing trench (Nance, 2008). Then the slabs of continental crust making up Laurussia and Gondwana began to collide, the earliest date of which has yet to be determined. This collision led to slight crustal thickening near the area of contact and the migration of deformation through previously weakened crust to the hinterland of the Variscan belt, leaving preserved in the rocks

10 deformational fabrics, faults, and medium-pressure metamorphic mineral assemblages. Then peak metamorphic conditions occurred: a metamorphic core formed fed by the melting crust beneath growing mountains at the collision zone. This core was 100s of km wide and it produced thermal pulses recorded as the Variscan granites that cut through the deformation fabrics and overprinted earlier evidence of lower-grade metamorphism as well as igneous dikes and veins. This core is what powered the large-scale regional low pressure-high temperature metamorphism of the surrounding crustal rocks including the prograde regional metamorphism of the Catalan Coastal Ranges (Fig 4). Finally, as the orogeny reached its end, the mountains grew too big, their mass too great, and the crust too soft for them to be supported. The orogeny entered its final stages of collapse and extension. With this collapse, granite was intruded fed by the lingering core, contemporaneous with extensional faults (Matte, 1991; Garcia-Navarro and

Fernandez, 2004).

The Guilleries Massif:

Located 60 km north of Barcelona and just south of the and the Pyrenean axial zone, but sharing a similar geology, the Guilleries massif is composed of Ordovician-aged black slates and shales metamorphosed in the biotite zone juxtaposed along Neogene faults with pelitic schists and orthogneisses metamorphosed above the sillimanite-cordierite-K-feldspar which record peak Variscan metamorphism. Rocks of the entire massif are in contact with

Ordovician-aged granite. Like the other Variscan massifs in Western Europe, the Guilleries massif has been intruded by and is nearly surrounded by Carboniferous S-type granites. The massif, like the entire Catalan Coastal Ranges, was later covered by Tertiary and Quaternary aged sediments (Sebastian et al., 1990) (Fig 5) and the massif rises abruptly above the surrounding Tertiary (65-1.8 Ma) cover.

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The Guilleries massif is composed of three major formations of metasedimentary and igneous rocks ranging in age from early Ordovician (approximately 480-470 Ma) to

Carboniferous (359 Ma-299 Ma). These formations from north to south are the St. Marti Sacalm formation, the Susqueda formation, and the Osor formation. All large structures and metamorphic zones are truncated by Neogene normal faults. Across the massif there is a metamorphic field gradient of low-pressure / high-temperature prograde regional metamorphism.

The metamorphic grade of the massif increases along a transect from north to south with chlorite and biotite grade rocks outcropping in the north and cordierite-K-feldspar grade rocks to the south (Duran, 1990) There is evidence of at least two deformational events, D1 and D2, preserved as two deformational fabrics in the massif. The oldest is S1, a south dipping, northeast-southwest trending fabric seen as slatey cleavage in the rocks of the lower metamorphic grade and visible mineral inclusion trails in the high-grade areas. The second fabric is S2. This is the dominant fabric of the medium- to high-grade rocks and is associated with east-west bearing lineations in schists and other metapelites (Julivert and Duran, 1990).

Areas of Low Grade Metamorphism:

The low grade found in the rocks of the St. Marti Sacalm formation is composed of slate, greywacke, limestone, and marble, and characterized by chlorite and biotite- andalusite grade metamorphism (Julivert and Duran, 1990) The formation is intruded and surrounded by granitoids. The youngest intrusion consists of leucogranitic dikes that crosscut the dominant deformation fabric of the massif dated ca 300 Ma (Martinez et al., 2011) and the oldest are voluminous Carboniferous S-type granites that surround the area with a crystallization age of ca. 319 Ma (Martinez et al., 2011).

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The St. Marti Sacalm formation is found above the chlorite-biotite isograd and characterized by the assemblage biotite-chlorite-muscovite. The St. Marti Sacalm preserves fossils, original sedimentary structures, and possesses a slatey cleavage. This cleavage is S1, defined by aligned , and records of the oldest deformation. S1 cleavage trends northeast- southwest and has a slight dip of 0-15 degrees to the south. In some areas a small crenulation cleavage cross cuts the S1 fabric (Julivert and Duran, 1990).

Areas of Medium- to High-Grade Metamorphism:

To the south of the St. Marti Sacalm formation is the Susqueda formation. The Susqueda formation is composed of a series of metapelitic schists, quartz veins, and a single horizon of marble which outlines large recumbent folds (Reche et al., 1998). The marble horizon is evidence that the depositional age of the Susqueda formation is late Cambrian to early

Ordovician (Duran, 1990). The Susqueda formation is characterized by its andalusite + cordierite and biotite + garnet + sillimanite + quartz prograde assemblages in pelitic and semi-pelitic schists and with some retrograde metamorphism in the biotite-chlorite zone. The

Susqueda formation is cut by andalusite-cordierite and sillimanite in its northern reaches and is located above the cordierite –K-feldspar isograd in its southern extremities. Intruded into pelitic schists of the Susqueda formation is the Susqueda Diorite with a crystallization age of 323

Ma (Riesco, 2004). The diorite and pelitic schist contact aureole reached pyroxene + garnet + cordierite grade (Riesco, 2004).

In the southern portion of the Susqueda formation relict in andalusite has been described indicating the reaction: staurolite + muscovite + quartz = biotite + andalusite + H2O

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(Duran, 1990). The predominate fabric in the Susqueda formation is S2. S2 originates from a second phase of regional deformation D2 which fully overprints S1.

Present in deformed pelitic schists of the Susqueda formation is garnet, particularly in quartzo-feldspathic rich zones. may have a poikiolitic texture and are often cracked. The garnet typically has mantles of cordierite and biotite (Sebastian, 1990). Garnet may also be reabsorbed by sillimanite and biotite. The reaction: garnet + muscovite = sillimanite + biotite + quartz has produced garnet pseudomorphs composed of sillimanite. These unstable garnets show internal inclusion trails parallel to S2 indicating syntectonic formation during D2 (Julivert, 1990).

The southern boundary of the Susqueda formation is marked by the appearance of the granoblastic Guilleries orthogneisses and the Osor formation. These orthogneisses are up to 300 m thick (Duran, 1990). While intruded by and truncated by younger rocks, the Osor formation is of Ordovician age. The Osor formation is primarily composed of pelitic schist with quartz veins with the highest grade assemblages characterized by sillimanite + cordierite + garnet + K- feldspar, the highest grade rocks in the massif. The Osor formation includes a horizon of ortho- approximately 50 m thick characterized by the assemblage ortho-amphibole + hornblende + (Sebastian, 1990). The protolith of the amphibolite is interpreted as late Variscan intrusions of mafic rock.

The Osor formation’s hallmark lithologies are biotite + garnet + sillimanite + quartz + K- feldspar +/- cordierite pelitic and semi-pelitic schists and gneisses. Relic staurolite occurs in andalusite (Duran, 1990). Also present are assemblages with cordierite + K-feldspar produced from the breakdown of biotite + sillimanite, possibly from the reaction: biotite + sillimanite + quartz = cordierite + K-feldspar + H2O. In the cordierite + K-feldspar metamorphic zone, there is

14 evidence of textural restructuring and the formation of not only S2 in both andalusite + cordierite and biotite + garnet + sillimanite + quartz + K-feldspar lithologies, but of a later phase of relaxation and extension stemming from a decrease in pressure as evidenced by the presence of relict staurolite included in muscovite suggesting a transition from staurolite + muscovite  biotite + aluminum rich + quartz (Sebastian et al., 1990)

The dominant S2 fabric in the Osor sequence is cross cut by a northeast trending dike swarm of peraluminous two-mica leucogranites (280-300 Ma) and surrounded by large intrusions of Carboniferous (319 Ma) aged granite (Reche et. al, 1998; Martinez et al., 2008). In some areas, S2 is statically overgrown by cordierite and K-feldspar suggesting peak thermal metamorphism occurred both syn- and post -tectonically with S2 deformation.

Relict micro fabrics preserved in the Osor and Susqueda formations, particularly inclusion trails in garnet, preserve the S1 deformational fabric. Therefore, the age of monazite found within S1 inclusion trails in the garnets of the Guilleries massif can reveal the dates of early deformation and metamorphism.

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Research Questions:

Garnets of the Guilleries massif preserve a least two rounds of deformation and metamorphism; older S1 fabrics preserved as inclusion trails are potentially Eo-Variscan having formed before the peak metamorphic conditions that affected the massif. It may be possible to determine the age of early deformation and metamorphism by dating inclusions that define S1 foliation preserved within garnet. This study asks the following four questions: (1) Are these inclusions relicts of an Eo-Variscan event that formed S1? (2) If so, what is the age of this early deformational event? (3) Is it, in fact, removed in time from peak Variscan metamorphism which is related to the intrusion of granites between about 325 and 300 Ma? And (4), Can isotopic ages offer insight into the timing of the Variscan orogeny away from the central collisional zone and metamorphic core?

In order to address these four questions, metapelitic samples from the Guilleries massif and their accompanying garnets are described, and U-Th-Pb ages of monazite included in garnet determined. During field work, three types of garnets based on their location and physical characteristics were collected and identified. Samples were subsequently thin-sectioned and garnet compositions determined through electron microprobe analysis to infer metamorphic conditions and history. Monazite present in inclusion trails was detected. Inclusions of monazite were dated using the U-Th-Pb method to infer the ages of deformational and metamorphic phases.

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Methods:

Sample Collection

Field work was done in June 2011. Samples were collected from road cuts along the main roads in the Guilleries massif (fig 6). Field relations, including descriptions of rock type, deformation structures (primarily foliations), , and cross-cutting relations of igneous rocks were recorded in the field using standard field methods.

Petrographic Analysis

Initially, standard uncovered polished thin sections of rocks collected in the field were examined using a Zeiss MC63S Axioplan optical microscope. The thin sections were prepared by Burnham Petrographics from one inch blanks. Also examined were thin sections of previously collected samples from the massif, selected because they contributed to the petrographic understanding of the region or because of the presence of garnets displaying unique inclusion patterns such as curved inclusion trails. Petrographic analysis was used to identify mineral assemblages, garnet inclusion patterns, metamorphic textures, S1 and S2 structural fabrics, and the mineralogy, nature, and extent of retrograde metamorphism.

Creating 1 inch Composite Mounts:

Epoxy mounts for ion probe analysis were prepared by cutting the thin sections into smaller pieces that included garnets with distinctive inclusion trails or potentially relevant mineral assemblages and fabrics, and mounting these thin section fragments in epoxy rounds to form composite thin section mounts. The mounts were created using 3M 701DL double-sided tape, a one inch plastic ring coated with Buehler 20-8185 release agent, and mixing an epoxy of

17 five parts (by weight) of Buehler 20-8130 epoxide resin with one part (by weight) of Buehler 20-

8132 epoxide hardener. To make the mounts, thin section portions were placed on the double- sided tape inside the coated one inch ring and newly created epoxy was poured into this form to a depth of roughly 10 mm. The mounts were left to cure for eight hours in a 50°C oven. The mounts were then hand polished until smooth and transparent using 1μm diamond paper. After polishing, the mounts were ultrasonically cleaned in soapy water and then rinsed with deionized water. After backscattered electron imaging, the samples were coated with gold. The gold coat provides an electrically conductive surface to make contact with the sample holder. These samples were coated with approximately 30 nm of gold using a sputter coater and then tested for conductivity. The conductivity of each sample was approximately 10Ω.

Backscattered Electron Imaging:

To confirm the presence of monazite, too small to see with the naked eye (20 μm across and smaller) in garnet inclusion trails, backscattered electron imaging (BSE) was used for detection. This study used a LEO 1430 VP Scanning Electron Microscope which has a 4- quadrant backscattered electron detector, an EDAX energy dispersive x-ray detector, and an

Oxford “mini-CL” cathodoluminescence detector at the University of California Los Angeles.

Samples were analyzed in situ, uncoated, and under variable pressure in one inch polished composite thin sections. Samples were imaged using an accelerating potential of 15 kV and current of ~30 nA. In BSE, monazite appears brighter than the surrounding due to the presence of high Z number thorium and rare earth elements within it (Cressey et al. 1999,

Harrison et. al. 1999, and Williams et. al. 2007). After identification of these bright spots in the garnets, they were analyzed for composition to avoid confusion with apatite or other minerals containing rare earth elements. The monazite grains selected for ion microprobe dating were then

18 photographed at 30X, 50X, 100X, 250X, and sometimes 500X depending on the size of the monazite grain selected. These photographs were used to record monazite location within the thin section and in relation to the host garnets, the metamorphic texture, structural fabrics, and other mineral phases. These photographs were then used to help locate the monazite grains for ion probe analysis.

Garnet Compositional Maps:

In situ x-ray maps of garnet composition, allowing for qualitative evaluation of garnet compositional zoning patterns, were created using the University of Kentucky’s Cameca SX-50 electron microprobe. Samples were mapped either in situ in one inch polished thin sections or in situ in standard one inch diameter epoxy ion microprobe mounts. X-ray maps of the concentration of calcium, , manganese, and were made from samples GU23B,

GU23BC, GILL85, and JRGA, JRGB, and JRGC. A 256 x 256 count scan with step sizes between 2 and 5 microns was made of all the samples except GILL85. The map of GILL85 was run overnight and produced with a scan count of 512 x 512. The beam current (the current generated by the filament in the gun chamber) for all the samples was 70 nA with sample current

(the current conduced to the ground) of 50nA.

Ion Microprobe Analysis:

In situ high-resolution ion microprobe geochronology of the U-Th-Pb series was performed using the Cameca IMS 1270 at the University of California Los Angles. After monazite location through backscatter electron imaging of the composite thin section mounts, these mounts were coated in gold. Prior to analysis on the probe, the mounts were examined with a reflected light microscope, the regions with monazite present were then located and

19 photographs for location reference were taken. The samples were then placed in the Cameca IMS

1270 overnight to reduce hydride background and create an ultra-vacuum environment. The ion microprobe uses a small beam, approximately 7μm in diameter with energy of 22.5 keV, to bombard the surface of a sample displacing secondary or backscattered ions and atoms through a collision cascade in a process called sputtering (Williams 1999, Stern 2009, Ireland

2003). These displaced ions are collected in a secondary column and the mass spectra of these secondary ions are resolved using a mass spectrometer. From these mass spectra the user is able to determine the ratios of specific isotopes in the sample. Isotope ratios after radioactive decay are analyzed to give the age of a sample. In order to resolve these masses, a magnet with a resolving power of approximately 5000 (M/ΔM) was used to eliminate peak inference from light rare earth element phosphate oxides (Harrison 1999).

In this study the samples were analyzed for 206Pb, 207Pb, 208Pb, 232Th, and 232U. The magnet in this study was cycled through the analyzed masses 7 times per minute and sample run for 40 minutes which results in multiple measurements of isotope ratios and allows for correction of beam stability. The average values of the measured isotope ratios were used to calculate the ages of the monazite. All analyses in this study used two monazite standards: 83-32 monazite

(Corfu 1988) with a 207Pb/206Pb age of 2685 Ma and a measured age of 2684 +/- 1 Ma

(MSWD=3.3, n=8) and a young secondary standard, Monazite 554 which is 45 Ma (Force 1997).

The calibration range for ThO2/Th was 0.179-0.404; UO/U=6.44-10.3 (n=8). All ages measured were corrected for common anthropogenic surface lead with the corrections shown in Sanudo-

204 232 ++ Wilhelm and Felgal (1994). Finally, all Pb counts were peak-stripped for ThNdO2 and the

232 143 ++ molecular isobar from counts measured at Th NdO2 .

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Chapter 2: Lithologies and Garnet Types

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The garnet-bearing pelitic schists of the Guilleries massif contain evidence of a polyphase metamorphic evolution. The metamorphic history of the massif is seen in the mineral assemblages, inclusion trails in garnets, in the chemical composition of the garnets, and in the ages of monazites.

Analyzed Samples:

GU23B: This sample was collected from a road cut in the pelitic schist of the Osor formation near leucogranitic intrusions and adjacent to a sill-like body of the Guilleries orthogneiss. The sampled outcrop shows a clear differentiation between quartz-rich psammitic layers and quartz-poor micaceous schist. The dominant foliation in the rocks is S2 defined by aligned biotite.

The matrix of sample GU23B is medium-grained and is composed of biotite, fibrolite

(sillimanite), quartz, and various opaques that are too fine-grained to be differentiated under a petrographic microscope. Staurolite is also present as degraded relicts in the matrix, indicating the initial prograde mineral assemblage included staurolite + muscovite + quartz; the peak metamorphic was sillimanite + biotite + garnet + water with quartz in excess (Duran, 1990). The bulk composition of the quartz-poor area that sample GU23B was taken from differs from the average (Shaw, 1956; Symmes and Ferry, 1992; and Ague, 1991) in that it shows lower Na and Ca, and nearly double the level of K (Martinez et al., 2008)

Retrograde metamorphism of this locality is recorded by mats of seriticized mica overgrowing garnet and biotite along S2. Some portions of biotite display a slight decussate texture with little evidence of shredding. Select individual quartz grains show undulatory extinction and an annealed texture indicative of recrystalization. S2 in the matrix of sample

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GU23B is parallel with the edge of garnet porphyroblasts, suggesting garnet is syn-tectonic with respect to D2 deformation.

Sample GU23CB: Sample GU23CB was collected from the same area of the Osor formation as sample GU23B. However, this sample was collected from a psammitic quartz-rich layer. The matrix is composed of approximate 85-90% quartz. These quartz grains have slightly lobate boundaries and some have undulatory extinction, possibly due to post-kinematic recrystalization. The minerals composing the rest of the matrix of GU23CB are biotite, fibrolite, and seriticized mica.

GILL85: Sample GILL85 is a plagioclase-rich layer similar in composition to that of sample GU23B. The mineral assemblage of the sample is plagioclase + garnet + biotite + quartz

+ fibrolite +/- cordierite, relict staurolite, and relict andalusite, recording high-temperature metamorphism of the Osor formation. Minerals of this high-temperature assemblage form a matrix parallel to S2. This matrix also contains fibrolite pseudomorphs of garnet from the reaction garnet + muscovite = biotite + silimanite + quartz. Cordierite envelops sillimanite and biotite from the reaction biotite + sillimanite + quartz = cordierite + K-feldspar + water; as before, these reactions and assemblages are all indicative of a high-temperature / low-pressure metamorphic regime (Spry, 1969), with temperatures reaching as high as 650 °C and pressures around 3.3-3.8 kb. Peak temperatures were calculated using garnet compositions and matrix biotite with the calibration of Spear, and Hodges (1982). Included in this high-temperature/ low-pressure assemblage is relict kyanite present in the matrix, recorded older, pre-S2 medium pressure metamorphism (Spry 1969).

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JRGA, JRGB and JRGC: As with sample GILL-85, samples JRGA, JRGB, and JRGC were collected from the plagioclase-rich layers similar in composition to sample GU23B in the

Osor formation, in an area intruded by ca. 300 leucogranites. The dominant fabric in samples

JRGA, JRGB and JRGC is S2.

The mineral assemblage of both samples is plagioclase + garnet + biotite + quartz + fibrolite +/- cordierite, relict staurolite, and relict andalusite, recording the high-temperature/ low-pressure regional metamorphism of the Osor formation. Preserved within the matrix of both samples are small relicts of kyanite, again recorded older, pre-S2 medium pressure metamorphism.

Garnet Types:

+2 +2 +2 +2 Garnet (X3Y2Z3O12 where the most common cations are X = Mg , Fe , Mn , and Ca ;

Y = Al+3, Fe+3; and Z = Al+3 and Si+4) is a robust mineral most often associated with metamorphic rocks. Garnet is known to retain the chemical record of its growth, its chemistry a function of changing pressure (P), temperature (T), and effective bulk composition (X) as a rock moves through multivariant P-T-X reaction space, with chemical and/or physical alteration occurring during retrograde conditions and fluid infiltration (Whitney et al., 1996). Furthermore, once formed, garnet generally behaves as a closed system with regards to its host rock matrix. In this manner, garnet will maintain and protect other metamorphic minerals that occur as inclusions within it, such as monazite, included during initial prograde metamorphism and subsequent evolution through multiple metamorphic and deformational episodes (Spear, 1993).

In this study, polished thin sections of rocks collected in the field were analyzed using an optical microscope to characterize their petrography and compositional maps of a variety of garnets

24 were created using a Cameca SX50 electron microprobe. Compositional zoning patterns in garnet help illustrate the thermal history of the sample (Spear, 1993; Kohn et al., 1997; Catlos et al., 2002; Williams et al., 2007).

Garnets from the Osor and Susqueda formations were examined and it was expected that they would contain monazite in their inclusion trails. Montel (1986) suggested that monazite will linger as inclusions due to its low rate of alteration and solubility during prograde metamorphism. Monazite will be preserved as inclusions in garnet because during metamorphism and garnet growth, monazite may not participate in reactions involving garnet.

Other minerals present in excess such as plagioclase, quartz, and biotite may also be present as inclusions (Passchier and Trouw, 1996 and Catlos et al., 2002). Harrison and Watson (1983) suggested that monazite inclusions may also result from the local presence and concentration of rare earth elements, thorium, and phosphate in excess during garnet growth.

Garnet bearing—garnetiferous (Wegman, 1995)--samples were analyzed from both the

Susqueda formation and the Osor formation. These two formations yielded three unique types of garnet: types A, B, and C each identifiable by their morphology, unique internal textures, and inclusion trails. Specific garnets were selected for analysis based on their shape, inclusion trail geometry, and the petrologic setting.

Type A: Type A garnets are defined by having inclusion trails parallel to S2 foliation defined by aligned minerals in the rock matrix (Fig 7). Samples GU23B and GU23BC both contain A type garnets. Type A garnets tend to be oblong with length-width ratios of about

2:1and in some cases they appear as crack fillings in the matrix. In Type A garnet, there are both cracks oriented perpendicular to the S2 foliation and cracks parallel to S2. Both sets of cracks are

25 filled with randomly oriented biotite and some retrograde chlorite. Often the cracks lead from the edge of the garnet to large biotite inclusions fully contained within the garnet; these elongate inclusions are parallel to S2. Inclusion trails in type A garnet contain monazite and are parallel to

S2, indicating growth of garnet synkinematic with respect to S2. Compositional maps of type A garnets reveal a homogenous internal composition without compositional zoning. This lack of zoning suggests that type A garnets were exposed to temperatures of 650 °C or greater resulting in the intracrystalline diffusion of Ca, Fe, and Mg, throughout the garnet (Passchier and Trouw,

1996) and chemical homogenization.

Type B: Type B garnets are defined by having sigmoidal-double spiral helicitic inclusion trails and poikiolitic textures. Sample GILL85 contains type B garnet (Fig. 8).

The Type B inclusion pattern may occur in two ways. First, when a garnet begins growth, it is syntectonic with foliation formation and captures inclusions parallel to this foliation; in the Guilleries massif, this foliation would be equivalent to S1. Then the garnet undergoes distortion and growth during a later metamorphic and deformational episode and captures foliation in a new orientation: in Guilleries, this episode would be equivalent to D2. Second, during growth, a garnet incorporates an existing foliation (in this case S1) syntectonically with the onset of a secondary deformational and metamorphic episode—D2 (Spry, 1969). As the second deformation progresses, the garnet with its inclusion trails along S1 will rotates along a newly created S2 fabric eventually producing the type B snowball garnet seen in sample GILL85.

Regardless of the formation process, Type B garnet preserves in its inclusion trails evidence of a pre-S2 deformation fabric.

26

Compositional maps of Ca in garnet G-GILL85 from sample GILL85 indicate the possible presence of a relict garnet core. This map also shows a slightly more Ca-poor rim that can be accounted for through retrogression and garnet resorption and the production of plagioclase. Outside of the Ca core, the compositional maps of Mg, Ca, and Fe show a homogenous compositional interior. Overall the compositional maps suggest heating to at least

650 °C, most likely during D2 (Spry 1969).

Type C: Type C garnets are defined by their slightly folded inclusion trails that are sub parallel to the external foliation, S2. In the Guilleries massif, the folded inclusion trails are interpreted as relict S1 foliation that has been refolded during D2. Type C garnets also display a range of textures from nearly perfectly idioblastic to poikiolitic. Garnets G-JRGA and G-JRGC in samples JRGA and JRGC, respectively, are examples of Type C garnet (Figure 9).

C type garnets are chemically homogenous in Ca, Mg, and Fe although garnet G-JRGA displays slight Ca depletion along its rim. Garnets G-JRGA and G-JRGC have muscovite and plagioclase coronas. Portions of the interiors of both garnets have been replaced with plagioclase. Staurolite occurs as inclusions within both garnets. Both garnets have obvious cracks, filled with biotite and chlorite, perpendicular to the dominant S2 foliation.

27

Chapter 3: Th-Pb Ages of Monazite

28

Monazite in garnet inclusion trails can be dated to elucidate the age(s) of metamorphism.

Monazite is appropriate for U-Th-Pb geochronology because it is a stable mineral which during crystallization incorporates relatively high concentrations of U and Th and very low levels of non-radiogenic Pb. Ten monazite grains that potentially could be dated were found within inclusion trails in Type A, B, and C garnets, and of these ten selected grains, six produced valid

U-Th-Pb ages (Fig 10). The rock samples containing the dated monazite inclusions are: GU23B,

GU23CB, GILL85, JRGA, JRGB, and JRGC. The ages reported are corrected for common anthropogenic Pb using 206Pb/204Pb = 18.86; 207Pb/204Pb = 15.62; 208Pb/204Pb = 38.34. (Sañudo-

Wilhelmy and Felgal, 1994). Isotopic data is presented in Table 1. All the monazite grains analyzed contained more than 95% radiogenic Pb, with the exception of JRGB, which contained

78.2% radiogenic Pb. The errors for the ages of all the samples analyzed are reported at the 1σ level. 232Th-208Pb ages were the most precise due to the standard used, and all ages, including

238U-206Pb and 235U-207Pb ages are concordant indicating that lead loss has not occurred, that is, the systems have remained closed to Pb since the time of monazite formation.

Results:

Samples GU23B and GU23CB: The garnets containing the dated monazite in samples

GU23B and GU23CB are small euhedral type A garnets. The monazite (M-GU23B) analyzed in sample GU23B presents a concordant 232Th-208Pb age of 312.8+/-7 Ma and the monazite (M-

GU23CB) analyzed in sample GU23CB has a concordant 232Th-208Pb age of 313.4+/-9 Ma.

These ages and their accompanying errors are within 1 σ of each other and are interpreted as the time of monazite growth.

29

Sample GILL85: The garnet containing the dated monazite in sample GILL85 is a Type

B garnet, with sigmoidal inclusion trails. The 232Th-208Pb age of the monazite is 334.1+/-8 Ma interpreted as the time of monazite growth that pre-dates the formation regional S2 foliation.

Samples JRGA and JRGC: The garnets containing the dated monazites in samples JRGA and JRGC are both Type C garnets, with inclusion trails of S1 that are folded into sub parallelism

232 208 with S2 in the rock matrix. Monazite in sample JRGA gives a Th- Pb age of 339.5+/-10 Ma and monazite in sample JRGC gives a 232Th-208Pb age of 341.3+/-8 Ma. Both of these ages are interpreted as the age of monazite growth that pre-dates for formation of regional S2 foliation.

Sample JRGB: The garnet containing the dated monazite (M-JRGB) in sample JRGB is also a C type garnet which is highly retrograded with a poikiolitic texture and a chemically homogenous interior. Monazite M-JRGB gave a 232Th-208Pb age of 393.3+/-18 Ma. This age is

Devonian age and likely the result of analytical error due to the low level, 78.2%, of radiogenic

208Pb in this particular monazite grain.

30

Chapter 4: Discussion

31

Dated monazites included in garnets of the high-temperature/ low-pressure regional metamorphic rocks of the Guilleries Massif present two distinct groups of ages that are interpreted to correlate with deformation and metamorphic phases. 1) Monazite with ages in the

Bashkirian Age of the Pennsylvanian from samples GU23B and GU23CB correspond with the

Carboniferous granitic intrusions associated with peak Variscan metamorphism and deformation.

2) Monazite inclusions with ages in the Visean Age of the Mississippian from samples GILL85,

JRGA, and JRGC, record monazite growth before peak Variscan metamorphism, and record in the Guilleries massif true Eo-Variscan deformation and metamorphism. This study supports the idea that this inboard massif underwent at least two phases of Variscan orogenesis, within 40 million years of each other, during the continental-continental collision of Laurussia and

Gondwana.

The monazites in samples GU23B and GU23CB gave 232Th-208Pb ages of 312.8 +/- 7 Ma and 313.4 +/-9 Ma, respectively. These ages correspond with peak Variscan metamorphism and the thermal event characterized by the widespread emplacement of Carboniferous-aged granites and low pressure/high temperature metamorphic assemblages containing sillimanite, cordierite, and K-feldspar. The monazite ages show that garnet and its inclusions formed syn-tectonically with peak-Variscan metamorphism and the S2 foliation that dominates the highest grade portion of the Guilleries massif. This thermal event is also recorded by the Variscan leucogranitic dikes that cross cut the massif with U-Pb ages of 305 +/- 1.9 Ma, 301.5 +/- 7 Ma, and 299 +/- 2.3 Ma

(Martinez et al., 2008), the regional Carboniferous (ca. 320 Ma) S-type granites that surround the massif, and the Susqueda diorite with a U-Pb crystallization age of 323.6+/- 2.8 Ma.

It is this thermal phase that created the regional low pressure-high temperature metamorphism of the Guilleries massif through magmatic heating. This regional high-grade

32 metamorphism is a classic Variscan feature but, what about the deformational stress acting upon the inland curst of the continent? Estrada and others (2002) suggested that D2 deformation stemmed from a compressional episode that is syn- and post-tectonic with the peak prograde metamorphism. Mineral assemblages show temperatures as high as 620-650 °C. (Sebastian et al.,

1990; Estrada et al., 2002). The syn-deformational nature of this metamorphism is supported by micro fabrics found throughout the massif. This D2 deformation could be associated with compression from the folding and faulting of the continental crust and the stacking of nappes leading to crustal thickening, and the formation of a metamorphic core that would continue to power the orogeny through its final stages, until gravitational collapse. Fibrolite mats overgrowing S2 micas in the samples collected suggest that this metamorphic event extended in time beyond the formation of the compressional D2 deformational fabric.

The D2 phase of deformation nearly erased all evidence of an Eo-Variscan deformation event in the high-grade rocks of the Susqueda and Osor formations. This earlier event, D1, created the S1 fabric seen in the lower grade rocks of the St. Marti Sacalm formation and is preserved in the inclusion trails of Osor and Susqueda garnets. Monazites M-GILL85, M-

JRG04A, and M-JRGC give 232Th-208Pb ages of 334.1 +/- 8 Ma, 341.3 +/- 8 Ma, and 339.5 +/-

10 Ma, respectively. These ages, with their maximum likelihood age of 338.5 +/- 5 Ma, all fall into the Visean Period (325-346 Ma) of the Mississippian epoch (359-318 Ma). This mean age suggests a deformation event before the intrusion of the Susqueda Diorite. This early event was most likely cold, non-magmatic, medium pressure deformation as evidenced by relict kyanite.

The monazite age data support the hypothesis of Martinez, and Estrada (2002) that the Guilleries massif records a high-temperature low pressure metamorphic event superimposed upon an earlier

Eo-Variscan low–temperature/ medium-pressure event.

33

Similar series of deformation and metamorphic events have been recorded, but not dated, in neighboring Variscan massifs in the Iberian Peninsula such as the Iberian massif and the El

Tormes thermal dome (Martinez, 1998, Gil-Ibarguchi, 1982). A better understanding of the

Variscan metamorphic and deformational progression and the crustal dynamics that accompanied the continental-continental collision can be achieved through dating Eo-Variscan signatures present in these nearby massifs. This study provides initial evidence that S1 fabrics preserved within garnets of the Guilleries massif are an Eo-Variscan feature and serves a as foothold in understanding how to proceed to learn more about the complete history of the Variscan orogeny.

34

Figures and Tables

35

Figure 1: A reconstruction of the Rheic Ocean, at its peak width, approximately 480 million years ago. Shortly thereafter subduction would begin near the -Carolina plate. Eventually this subduction would lead to a continental-continental collision closing the Rheic Ocean and creating the Variscan Orogeny. Image after Nance 2008.

36

Figure 2: Location of the isolated Variscan Massifs in Western Europe. These areas are the portions of Europe where the results of the Variscan orogeny are visible today in the form of high grade metamorphic rocks and unique deformational fabrics. The study area is located in the Catalonian Costal Ranges marked in red. This terrain is south of Pyrenean which is a late Variscan deformational feature. The Variscan Front, the demarcation of the extent of the deformation and metamorphism from the Variscan Orogeny, is marked in a thick dashed line.

37

Figure 3: Global map showing the extent of the Variscan outcrops in the modern day. Today the high-temperature low pressure metamorphic rocks of the Variscan Orogeny can be seen from the Appalachian and Ouachita Mountains of Eastern North America, the of Northwestern Africa, to the of Russia. In Western Europe the Variscan outcrops are persevered in isolated massifs.

38

Figure 4: The Catalan Coastal Ranges in Northeastern Spain. These mountains display low pressure - high temperature Variscan metamorphism. Partially covered by Tertiary and Quaternary sediments are large swaths of Carboniferous age granite surrounding pelitic schists and orthogenesis. Also present are shales and slates ranging in age from the Ordovician to the Carboniferous.

39

Figure 5: a.) Map showing the simplified rock and stratigraphic relationships of the Guilleries Massif. Isograds are represented with think dashed lines. The isograd location is approximate and does not take into consideration the location of plutons. From the North to South the isograds increase in metamorphic grade and are Biotite grade, Silliminate grade, and lastly Cordierite—K- feldspar grade. Visible are the unconsolidated Tertiary and Quaternary sediments which surround the massif. In the Northeast there is a small outcrop of limestone in which Devonian (417- 354mya) pelagic limestone is found (Duran 1990). This limestone contains fossil brachiopods and trilobites. The Biotite isograd crosses the St. Marti Sacalm Formation composed of Ordovician grey slates and greywackes; there are small layers of rich conglomerates and volcanic ash. Stratigraphically below the St. Marti Sacalm formation and bisected by the Silliminate isograd is the Susqueda formation. A series of Ordovician rocks with highly metamorphosed slates and pelitic schists with quartz veins. Separating this unit from the stratigraphically lower Osor unit are the Guilleries Gneisses. Fine grained Ortho-gneisses of early Ordovician age. The stratigraphically lowest unit is the Osor Formation with its pelitic schists and amphibolite horizon all of late Cambrian to Early Ordovician age. The S-type granite which surrounds the older Pelitic rocks has a crystallization age of ca 300 mya while the Leucogranites which intrude these rocks have been dated to ca 319. The Susqueda diorite crystallized 323 mya. b) A rudimentary stratigraphic column showing the extent of the isograds. Metamorphic grade in the massif increases from north to south.

40

Figure 6: Map of sample collection areas within the Guilleries Massif. Six samples were collected from medium-high grade garnet bearing pelitic schists of the Susqueda and Osor formations.

41

Figure 7: Type A garnets, identifiable by their S2 parallel inclusion trails. These garnets are found in the Osor formation in the both the quartz rich and quartz poor layers. The of the sample, Qtz + Bt_+ Fib, suggests a medium to high temperature-low pressure metamorphic history. Compositional maps reveal homogenous textures indicating exposure to high (650+ °C) temperatures. Horizontal cracks filled with Chl and Bt suggests retrogression.

42

Figure 8: Type B garnet, characterized by sigmoidal inclusion trails that preserve S1 fabric. G-GILL85 sits in a fabric composed of Plag + Grt + Bt +Qtz + Fib +/ Crd, relict Str, and And. Relict kyanite is present preserved in patches in association with muscovite. The matrix fabric is S2.

43

Figure 9: Type C garnets are identifiable by their sub parallel inclusion trails. These inclusion trails most likely preserve S1. These garnets display a largely homogenous chemical composition. Though there is slight Ca rimming suggesting diffusion at a high temperature. These garnets sit in a Plag + Grt + Qtz + Fib + Bt matrix with the dominant foliation S2.

44

Figure 10: Timeline showing the distribution of ages obtained from microprobe dating and other major Variscan events. Contained in the red boxes are the Th-Pb dates obtained for this study. Blue and grey boxes with solid lines contain ages previously obtained in other studies. All ages are in millions of years and are known to be concordant. The Variscan Leucogranites have U-Pb ages of 305 +/- 1.9 mya, 301.5 +/-7 mya, and 299.0 +/- 2.3 mya. (Martinez et al. 2008). The Susqueda Diorite exhibits an U-Pb age of 323.6+/- 2.8 mya (Martinez et. al. 2008). Lastly there is deformed Ordovician granitic bedrock present in the region with ages ca. 480 mya (Martinez et. al. 2011)

45

208 232 206 238 207 23 Sample Age (Ma): Th O2/Th % Age (Ma): Age (Ma): Age (Ma): U O/U % % Pb*/ Th Pb/ Pb/ 208Pb/232Th Radiogenic 206Pb/238U 207Pb/206Pb 207Pb/206Pb Radiogenic Radiogenic U 5U 208Pb 206Pb 207Pb

0.0156+/- 0.04144+ 0.2634+ GU23B 312.8 +/- 7 0.43 98.5 262+/-29 3 266 9.36 96.0 57.0 0.0004 /-0.0047 /-0.042

0.01563+/- 0.04342+ 0.2331+ GU23CB 313.4+/-9 0.32 99.3 274+/-28 * * 9.59 90.5 30.7 0.0004 /-0.0045 /-0.065

0.01667+/- 0.04897+ 0.3501+ GILL85 334.1+/-8 0.40 97.5 308+/-21 279 31 10.85 99.1 87.6 0.0004 /-0.0034 /-0.025

0.01694+/- JRGA 339.5+/-10 0.31 98.8 * 179 169 0.0402 95.4 54.9 0.0005 * * 0.05663+ 0.01965+/- /- 0.2746+ JRGB 393.3+/-18 0.34 78.2 355+/-77 * * 8.95 56.2 5.3 0.0009 0.00127 /-0.551

0.01703+/- 0.04573+ 0.3504+ JRGC 341.3+/-8 0.37 98.0 288+/-30 435 410 9.61 89.7 36.5 0.0004 /-0.0048 /-0.077 Table 1: Th-Pb and U-Pb ages from monazite inclusions in garnets from the Guilleries Massif. These data were collected using a Cameca IMS1270 ion microprobe. The primary beam an 16O with a total impact energy of 22.5 KeV. A sample HV (V) of 10,000. A transfer lens, contrast aperture, and field aperture of 150, 400, and 3000 μm respectively. The energy window was +10 to -40 (V), with the exception of 208Pb, Th, and

ThO2 which was -20 to -70 V. The detection for collection was set in the electron multiplier pulse counting mode with 7 counts a minute and each sample was run for 40 minutes. The mass resolving power was at 5000 (M/ΔM) All calculated error in the table is within one sigma. The data for boxes marked with an asterix was not obtained. All ages are concordant.

46

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47

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