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Constraining the age of metamorphism in the Dora Maira, Western Alps: Implications of U-Th-Pb ages and REE concentrations in phosphates

A Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston

In Partial Fulfillment of the Requirements of the Degree Master of Science

By Katherine Tilghman December 2014

Constraining the age of metamorphism in the Dora Maira, Western Alps: Implications of U-Th-Pb ages and REE concentrations in phosphates

______Katherine Tilghman

Approved: ______Dr. Thomas Lapen ______Dr. Virginia Sisson ______Dr. Alexander Robinson ______Dr. James Meen

______Dean, College of Natural Science and Mathematics

Acknowledgements

I would like to thank my committee chair, Dr. Tom Lapen, for his patience, constructive feedback, and willingness to explore alternative hypotheses. I would also like to thank my committee members, Dr. Jinny Sisson, Dr. Alex Robinson, and Dr. Jim

Meen, for their knowledge and wisdom throughout my time as graduate student at

University of Houston.

Thanks also go to my friends and colleagues and the department faculty and staff for making my time at University of Houston a great experience.

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Constraining the age of metamorphism in the Dora Maira, Western Alps: Implications of U-Th-Pb ages and REE concentrations in phosphates

A Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston

In Partial Fulfillment of the Requirements of the Degree Master of Science

By Katherine Tilghman December 2014

ABSTRACT

Understanding the rates of subduction, burial, and exhumation requires accurate and precise age determinations of various stages along a unit’s pressure-temperature path.

Advances in geochronology result in age uncertainties that are significantly less than the duration of many geological processes such as prograde metamorphism and exhumation.

Thus, it is more important than ever to precisely link the measured ages to the metamorphic conditions. This study presents U-Th-Pb and rare earth element data collected by LA-ICPMS for monazite, monazite-apatite symplectite, and florencite in pyrope quartzites from Parigi,

Dora Maira nappe, western Alps, Italy. U-Pb and Th-Pb isochron ages of the monazite are

31.48 ± 0.22 and 32.67 ± 0.70 Ma, respectively (all uncertainties are reported at 2σ). The symplectite (+florencite) yielded U-Pb and Th-Pb isochron ages of 31.3 ± 1.7 and 31.8 ±

4.3Ma, respectively; these phosphate mineral assemblages are indistinguishable in age and initial Pb isotope composition from the monazite. The monazite ages of this study are younger than ages of peak metamorphic conditions as represented by a Lu-Hf garnet-matrix age of 34.1 ± 1.0, recalculated with 176Lu decay constant, and a 35.1 ± 0.9 Ma U-Pb age of

UHP titanite. Monazite ages of this study are consistent with, but slightly younger than U-

Th-Pb monazite ages from previous studies and in agreement with U-Pb ages of retrograde titanite. Given that the monazite from this study likely records REE liberated by garnet break-down associated with the reaction phengite+pyrope+H2O = phlogopite+kyanite+talc, ages from this study likely reflect the timing of this process. The P-T conditions of this reaction are estimated to lie above the quartz-to-coesite transition, so the monazite ages may

record UHP conditions of the retrograde path. Given these constraints, initial exhumation rates of the Dora Maira nappe may be significantly greater than the 3.4 cm/yr previously proposed.

TABLE OF CONTENTS

Project 1 1 Introduction 1 1.1 Background 3 1.2 Geologic Setting 5 1.2.1 Units of the Dora Maira 8 1.2.2 Pyrope Whiteschist Protolith 10 1.3 Previously Reported Age Constraints 12 2 Methods 2.1 Electron Probe Microanalysis and Scanning Electron Microscopy 14 2.2 LA-ICP-MS Spectrometry 16 3 Results 3.1 Petrography 17 3.1.1 UHP Phase Assemblage 19 3.1.2 Retrograde Phase Assemblage. 22 3.1.3 Minor Phase Assemblage 24 3.1.4 Florencite 27 3.2 Trace Element Compositions 31 3.2.1 Phosphates 31 3.2.2 Garnet 38 3.3 Ages 39 4 Discussion 4.1 Phosphates 44 4.1.1 Florencite 44 4.1.2 Monazite Apatite Symplectite 45 4.1.3 Ages 46 4.2 Implications of REE Concentrations 47 4.3 P-T-t Path 49 4.3.1 UHP Terranes 49 4.3.2 Monazite 50 4.3.3 Exhumation Rates 50 5 Conclusions 52 6 References 63

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Project 2 1 Introduction 54 2 Methods 2.1 Trace Element Compositions 54 2.2 Lu-Hf and Sm-Nd 55 3 Results 3.1 Ages 58 3.2 REE Compositions 58 4 Discussion 4.1 Sm-Nd system 61 5 Conclusion 62 6 References 63

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LIST OF FIGURES

Figure 1 Geologic Map of the Western Alps 4 Figure 2 Subduction and Exhumation Model 7 Figure 3 Simplified Geologic Map of the Southern Dora Maira Nappe 11 Figure 4 Cross Section Through the Southern Dora Maira 12 Figure 5 Previous Ages 14 Figure 6 Petrography- General Matrix 18 Figure 7 Petrography- Garnet inclusions 20 Figure 8 Petrography- Coesite 21 Figure 9 Petrography- Talc Rim Forming Around Garnet 23 Figure 10 Petrography- Elongated Quartz 24 Figure 11 BSE images Apatite and Monazite 26 Figure 12 Petrography- Florencite 28 Figure 13 BSE images Florencite 29 Figure 14 Element Maps of Florencite 30 Figure 15 Combined Element Maps of Florencite 31 Figure 16 Trace Elements for Monazite 35 Figure 17 Garnet REE Concentrations 38 Figure 18 Isochron Diagrams for Unaltered Monazite 40 Figure 19 Isochron Diagrams for Altered Monazite, Florencite, and Apatite 41 Figure 20 Concordia Diagram 42 Figure 21 P-T-t Diagram 52 Figure 22 Sm-Nd Concordia Diagram 59 Figure 23 Lu-Hf Isochron 59 Figure 24 Garnet Rim Lu-Hf & Sm-Nd data 60

Table 1 Electron Microprobe Analysis of Monazite 33 Table 2 LA-ICP-MS wt.% oxides for Monazite 34 Table 3 LA-ICP-MS ppm for Monazite 34 Table 4 Electron Microprobe Analysis of Apatite 36 Table 5 Electron Microprobe Analysis of Florencite 37 Table 6 Ages and Pb Isotope Data 44 Table 7 Leaching Methods 56 Table 8 Sm-Nd Element and Isotope Data 57

1 Introduction

Coesite-bearing units within the Dora Maira nappe, south-western Alps, Italy

(Figure 1), represent the first recognized occurrence of continental crust subduction to depths greater than 100 km and resultant ultra-high pressure metamorphism (UHP). The high P/T metamorphic field gradient and rapid exhumation (e.g. Chopin, 1984; Schertl et al., 1991; Rubatto and Hermann, 2001) of these units allow insights into the nature and processes of continental crust subduction without late thermal overprinting that often obscures the high P-T mineral assemblages (e.g. McClelland and Lapen, 2013). The subduction of continental crust to UHP conditions is typically associated with a transition from ocean-continent to continent-continent collision as a remnant ocean basin collapses.

The rapid exhumation of units such as the Dora Maira have been modeled to result from lithospheric slab break-off, buoyancy-driven forces, wedge thickening, and faulting

(Michard et al., 1993; Chopin, 1997; Ernst et al., 1997; Ernst, 2001) and typically a combination of these processes. Typically, the exhumation rates for UHP terranes worldwide are rapid, ranging from 2.0- 3.4 cm/yr (Gebauer et al., 1997; Rubatto and

Hermann, 2001).

A constraint on the exhumation rates of UHP units is difficult because of the short time interval of the process and the, oftentimes, difficulty in relating measured ages to their corresponding P-T conditions. In pyrope whiteschists of the Dora Maira, a number of minerals are found as inclusions in the pyrope garnets that indicate UHP metamorphic conditions. The dating of these minerals (e.g. garnet, monazite, titanite) help constrain the timing of peak metamorphism. For the Dora Maira, the calculated P-T path indicates

that the time of peak P should be nearly coeval with the time of peak T (e.g. Schertl et al.,

1991), thus the peak T assemblage should represent near peak P conditions. The age of peak metamorphism, based on a Lu-Hf isochron of garnet-matrix and U-Pb in titanite is

34-35 Ma (Duchene et al., 1997; Rubatto and Hermann, 2001), but there is less constraint on the retrograde path of the Dora Maira. In addition to mineral assemblages seen within the sample, the only ages helping to constrain the Dora Maira's retrograde path are zircon fission track ages (Gebauer et al., 1997), titanite ages (Rubatto and Hermann, 2001), and phosphate ages (this study).

This study aims to constrain the cooling and decompression histories of the Dora

Maira nappe through the analysis of phosphate mineral assemblages of pyrope whiteschists from Parigi, western Alps, Italy. Rare earth element (REE) concentrations and textures of monazite, monazite-apatite symplectites, and florencite indicate that retrograde garnet breakdown reactions are recorded in these minerals. Dating of these phosphates thus links the measured ages to a segment of the P-T path associated with garnet break-down reflecting the cooling and decompression of the Dora Maira nappe.

Combining these trace element concentrations with the age data allows the timing of garnet breakdown to be constrained and further constrains the early stages of exhumation.

Using UHP ages and retrograde phosphate and titanite ages a more accurate determination of exhumation rates can be achieved. Further, during retrograde metamorphism, the Dora Maira did not have a constant exhumation rate as previously assumed (Rubatto and Hermann, 2001). The varying exhumation rates within the Dora

Maira can also help constrain subduction and exhumation models. These slow

exhumation models of UHP terranes which experience steady cooling followed by some heating (Ernst et al., 1997; Grasemann et al., 1998) and fast exhumation models of UHP terranes which are characterized by rapid crustal cooling with little to no heating and isothermal decompression (Ernst et al., 1997; Grasemann et al., 1998). Because the results of my research, I propose a new exhumation model for the Dora Maira, with varying exhumation rates, which improves the understanding of continental crust subduction and exhumation where UHP units experience rapid cooling.

1.1 Background

The Dora Maira nappe, first mapped by Vialon in Parigi, is a narrow belt of crystalline Paleozoic terrains extending over 70 km in the western Alps (Vialon, 1966;

Chopin, 1984). The Dora Maira, along with the Monta Rosa and Gran Paradiso, make up the three internal crystalline nappes in the western Alps. It consists of several continental crustal slices which differ in metamorphic grade and are separated by major low-angle faults. A number of minerals (bearthite, coesite, ellenbergite, and magnesiochloritoid) are found as inclusions in the pyrope garnets that indicate HP to UHP conditions. Ultra-high pressure metamorphism occurs at pressures in the coesite stability field; it is indicated by the presence of diamond or coesite and has an implied depth of > 100 km (Gilotti, 2013).

The Dora Maira displays a steep P-T path with a rapid increase in temperature and pressure during the UHP peak metamorphic conditions, and the retrograde P-T path is along a steep P-T gradient (Chopin, 1987; Schertl et al., 1991; Rubatto and Hermann,

2001; McClelland and Lapen., 2013;).

Figure 1: Geologic map of the Western Alps. The Austroalpine nappe system is made up of extensional allochthons between the Adriatic margin and the Alpine Tethys and has a Eoalpine metamorphic overprint. The Helvetic Zone consists of deformed cover rocks and nappes in the northern Alpine foreland. The Piedmont Zone represents the Western part of the Jurassic Piedmont Liguria Ocean and consists of blueschist and the Zermatt-Saas ophiolite. The Dora Maira (DM), Gran Pardiso (GP), and Monte Rosa (MR) are all domal structures that make up the Inner Penninic Nappes. These nappes are below the Piedmont zone, but are exposed at the surface. (Bousquet et al., 2012, Tectonic framework of the Alps)

1.2 Geologic Setting

The western Alps run north-south, forming an arc around the Po Basin, and are composed of three separate units: Helvetic, Inner Penninic, and Austroalpine (Figure 1).

The Helvetic unit is made up of sedimentary rocks that are part of the European continental margin. The Penninic domains are ocean-continental tectonic units. The continental units of the Penninic domain include the three crystalline basement nappes of the Dora Maira, Gran Paradiso, and Monte Rosa; while the Piedmont Zone is comprised of Mesozoic sediments that were deposited in marine basins of the Tethys Ocean between the European and Adriatic plates (Ernst et al., 1999; Compagnoni, 2003; Cosca et al.,

2005; Pfiffner, 2014). The Austroalpine unit is part of the Adriatic continental margin.

The Penninic unit lies structurally on top of the Helvetic unit, and below the Austroalpine unit (Pfiffner, 2014). The Dora Maira, Gran Paradiso, and Monte Rosa are three basement-cored nappes that are part of the Inner Penninic Domain (Figure 1), and represent the former eastern edge of the European plate that has been subducted and then exhumed (Gebauer et al., 1997; Chopin et al., 1999; Gasco et al., 2011; Figure 2). These nappes are interpreted here to be crystalline basement derived from the Briançon microcontinent (e.g. Dal Piaz, 1999; Lapen et al., 2007).

The Alpine orogeny was mainly active between 65 Ma and 2.5 Ma but is still active today. During the Late Cretaceous to Late Eocene, the nappes were stacked and intensely folded (Pfiffner, 2014). The Dora Maira experienced late Eocene – early

Oligocene UHP metamorphism (coesite-eclogite) with estimates of peak metamorphic conditions of 700-800° C and 27-35 kbar (Chopin et al., 1991). The coesite-bearing

monometamorphic unit of the Dora Maira consists of continental crust originally composed of Variscan metamorphic basement that was intruded at 275 Ma by granitoids

(Compagnoni and Rolfo, 1991).

The western Alps record Cretaceous to present convergence between the African continental upper plate and the subducting European lower plate. The relatively low density European plate was subducted to depths of 100 km beneath the African plate and then rapidly exhumed, forming the present day nappes (Chopin, 1987). Molnar and Gray

(1979) determined that continental crust could be subducted between 10 and 300 km.

The negative buoyancy of the cold mantle lithosphere and the pull of the oceanic down going slab can cause such continental subduction. The exhumation of the Dora Maira can be achieved by lithospheric slab break-off, buoyancy-driven forces, wedge thickening, and faulting. The break off of the Piedmont oceanic lithosphere caused the

Dora Maira to decouple from the descending lithospheric plate and move back up the subduction channel. Underplating thickened the wedge, caused the Dora Maira to extend internally, and results in the uplift of the UHP rocks. Thrust faults present within the

European plate near the subduction zone migrated westward as subduction continued.

The migration of thrust faults, in addition to buoyancy-driven processes, helped with the rapid exhumation of the ultra-high pressure rocks (Figure 2). The varying ages and grades between the units within the Dora Maira were achieved by the migration of intra- continental thrust faults (Chopin, 1987; Michard et al., 1993; Henry et al., 1993). The underthrusting of cold units below the overthrusted higher metamorphic grade units may

explain the lack of increasing isotherms in the higher grade units and the preservation of

UHP minerals (Chopin, 1987).

Figure 2: Tectonometamorphic evolution of the western Alps with the convergence of the African and European plates during the Cretaceous. Thrust faults present within the European plate near the subduction zone migrated westward as subduction continued. The migration of thrust faults, in addition to buoyancy driven processes, helped with the rapid exhumation of the ultra-high pressure rocks. DM=Dora Maira (Modified from Pfiffner, 2014).

The following section highlights, in chronological order, the major tectonic events involved in the evolution of the Dora Maira nappe during the Cretaceous to present

(Figure 2).

1. During the Mid-Cretaceous (~92 Ma) the African plate started moving north resulting in a south-dipping subduction zone between the Piedmont Ocean and the

Adriatic margin, with the Piedmont ocean subducting beneath the Adriatic continental plate (Pfiffner, 2014).

2. During the Eocene, subduction of the European plate stopped after detachment of the oceanic lithosphere from the continental crust (Pfiffner, 2014). The Inner Penninic units (Dora Maira, Monte Rosa, and Gran Paradiso) underwent UHP to HP metamorphism in the Eocene (Meffan-Main et al., 2004; Lapen et al., 2007). The Dora

Maira experienced ultra-high pressure, low-temperature metamorphism (37 kbar, 800° C)

(Chopin, 1984; Avigad, 1992; Pfiffner, 2014).

3. In the Late Eocene-Early Oligocene, the Piedmont oceanic crust became sandwiched between Adriatic and European crust, causing the formation of a thick orogenic wedge. The coesite-bearing unit of the Dora Maira moved upward and began exhumation (Avigad et al., 1993; Pfiffner, 2014).

1.2.1 Units of the Dora Maira

The Dora Maira nappe is a narrow belt of crystalline Paleozoic terrains extending over 70 km, running roughly north-south. The Dora Maira extends from Val Susa in the north to Val Maira in the south, and is bound by the Po basin on the east (Chopin et al.,

1999). The Dora Maira consists of a Paleozoic basement, composed of a volcano- sedimentary series with an amphibolite facies overprint and a Mesozoic cover (Chopin,

1984; Gebauer et al., 1997). It structurally overlies a metasomatized Hercynian granitic

basement rock, which is the protolith of the whiteschist lens in the UHP unit (Gebauer et al., 1997; Chopin et al., 1999). The Dora Maira is made up of polymetamorphic and monometamorphic units. The polymetamorphic complex consists of gneisses, micaschists, marbles, and eclogites. The monometamorphic complex consists of mylonitic orthogneiss and the pyrope whiteschists, which occur in the orthogneiss

(Compagnoni and Rolfo, 1991; Ferrando et al., 2009). The whiteschist lens and orthogneiss are separated by a phengite-rich banded gneiss that ranges from several centimeters to several meters in width (Ferrando et al., 2009).

The Dora Maira nappe, first mapped by Vialon in Parigi, is folded into a half- dome that dips west to south (Vialon, 1966; Chopin, 1984; Figure 3). It consists of several continental crustal slices differing in metamorphic grade and are separated by major low-angle faults (Figures 3 & 4). The three major units are the Brossasco-Isasca

Unit, Dronero Unit, and Pinerolo Unit. Each unit reached different depths within the alpine orogenic wedge, indicated by different peak pressure -temperature conditions for each unit. The Brossasco-Isasca unit consist of late Paleozoic granite that has been transformed to orthogneiss and metasedimentary rocks. This unit is the core of a recumbent fold, and also consists of quartz-rich kyanite garnet schists with lenses of pyrope-quartzite. The pyrope-quartzite lens contains coesite (Chopin, 1984; Chopin,

1987). This layer has pyrope garnet nodules that vary in size, ranging from 0.2 cm to 25 cm (Gebauer et al., 1997). The Brossasco-Isasca unit experienced coesite – eclogite facies metamorphism, at temperatures of 700-800 °C and pressure of 28-35 kbar (Chopin,

1984; Chopin et al., 1991). The second unit is the Dronero Unit, also referred to as the

‘cold eclogite unit.' The Permian- Triassic Dronero unit is a sequence containing metagranites, schists and metapelites (Avigad, 1992). It has undergone eclogite facies metamorphism at 12-20 kbar and 500-550 °C (Chopin, 1984; Chopin, 1987; Chopin et al., 1991). It has experienced lower temperatures and pressures than the adjacent

Brossasco-Isasca unit. The lowest unit, the Pinerolo, consists of Permo-Triassic strata, mica schist, and gneiss, that have been intruded by granitic bodies and have a polymetamorphic history. It has undergone epidote-blueschist facies metamorphism at

10-12 kbar and 500-550 °C.

1.2.2 Pyrope Whiteschist Protolith

There are two proposed origins of the magnesium-rich whiteschist: 1) an evaporitic sedimentary origin where whiteschists are high grade metamorphic equivalents of Mg-rich pelites (Chopin, 1984; Schertl et al., 1991; Philippot, 1993; Schreyer et al.,

1997) and 2) pre-Alpine hydrothermal metasomatism along shear zones (Chopin et al.,

1991; Sharp at al., 1993; Philippot et al., 1995; Demeny et al., 1997). In the Dora Maira there is a close field association of orthogneiss and whiteschist that supports the metasomatic origin (Compagnoni and Hirajima, 2001). The whiteschists are derived from a granitoid protolith that has undergone metasomatic processes that occurred along shear zones in the presence of metasomatic fluids during prograde UHP evolution

(Compagnoni and Rolfo, 1991; Compagnoni and Hirajima, 2001; Ferrando et al., 2009).

It is widely accepted that the whiteschists have undergone Mg-rich metasomatism.

Dehydration of the subducting European plate began at shallow levels, and Mg-rich

fluids evolved from the breakdown of hydrous phases (Sharp et al., 1993).

Metasomatism of the orthogneisses along shear zones by magnesium-rich fluid helps explain the chemistry of the Dora Maira's whiteschist and the nearly end-member pyrope garnets. Zircon sampled from the pyrope quartzite lens in the Dora Maira yield a U-Pb age of 275 Ma; this may reflect the age of magmatic crystallization (Gebauer et al.,

1997).

Figure 3: Simplified geologic map of the Southern Dora Maira Nappe. Modified from Avigad et al. ( 1993); Henry et al. (1993); and Zechmeister et al. (2007).

Figure 4: Cross-section through the southern Dora Maira and the overlying units. The Dora Maira Nappe consists of several continental crustal slices differing in their metamorphic grade that are separated by low-angle faults. The three major units are the Brossasco-Isasca Unit (BIU), Dronero Unit (DU), and Pinerolo Unit (PU). The BIU is the core of a recumbent fold that is composed of quartz-rich kyanite garnet schists with lenses of pyrope-quartzite. The Dronero unit (DU) is a detrital sequence with metagranites, schists and metapelites. The Pinerolo unit (PU), consists of Permo-Triassic Strata, polymetamorphic, mica schist, gneiss, metabasites that have been intruded by granitic bodies. Modified from Avigad et al. ( 1993); Henry et al. (1993); and Zechmeister et al. (2007).

1.3 Previously Reported Age Constraints

Previous studies from the Dora Maira have characterized the metamorphic evolution of subducted continental crust and have yielded varying ages for UHP metamorphism since Chopin’s discovery of coesite in 1984. The reported age of UHP metamorphism is 35-38 Ma based on zircon (U-Pb), garnet (Lu-Hf, Sm-Nd), ellenbergite

(U-Pb), and monazite (U-Pb) (Tilton et al., 1989; Tilton et al., 1991; Duchene et al.,

1997; Gebauer et al., 1997; Figure 5). The range in ages is extended by Paquette et al.

(1989) who interpreted the age of UHP metamorphism to be 121 (+ 12/ - 29) Ma, by U-

Pb dating of zircon in pyrope quartzites, but this age is at odds with most other geochronological constraints. Tilton et al. (1989) dated garnet using Sm-Nd, indicating the age of UHP metamorphism as 34-38 Ma; while U-Pb dating of zircon yielded ages of

50-55 Ma. In 1991, Tilton et al. expanded their Sm-Nd and U-Th-Pb isotope dataset to refine the age of UHP metamorphism to 35-38 Ma and 38 ± 1.4 Ma, for Sm-Nd and U-

Th-Pb, respectively. The pyrope whiteschists from the Dora Maira gave a Lu–Hf garnet- whole rock age of 34.1 ± 1.0 Ma (Duchene et al., 1997). In situ U-Pb SHRIMP ages of zircon, which are more difficult to assign to a particular set of metamorphic conditions, are 35.4 ± 1.0 Ma (Gebauer et al., 1997). In situ U-Pb data of monazite gave an age of 35

± 7 Ma (Vaggelli et al., 2006). These data are significantly older than monazite ages of

32.2 ± 0.1 Ma and 32.1 ± 0.1 Ma using 208Pb/232Th, and 34.3 ± 0.3 Ma and 34.1 ± 0.7 Ma for 206Pb/235U (Tilton et al., 1991). Rubatto and Hermann (2001) dated titanite using U-

Pb, yielding an UHP age of 35.1 ± 0.9 Ma, and U-Pb ages of retrograde titanite of 32.9 ±

0.9 Ma and 31.8 ± 0.5 Ma. All together, estimates for the age of UHP metamorphism ranges from 34-38 Ma (Tilton et al., 1989; Tilton et al., 1991; Duchene et al., 1997;

Rubatto and Hermann, 2001; Vaggelli et al., 2006). However, the age of UHP metamorphism is best represented by chronometers that reflect growth and/or equilibration at the peak P-T conditions. These include the 34.1 ± 1.0 Ma Lu-Hf garnet- matrix age (Duchene et al., 1997, recalculated with the 176Lu decay constant of Scherer et al., 2001) and the 35.1 ± 0.9 Ma U-Pb age of UHP titanite (Rubatto and Hermann, 2001).

Figure 5: Compilation of previously reported ages for UHP metamorphism of the Dora Maira nappe.

2 Methods

2.1 Electron Probe Microanalysis and Scanning Electron Microscopy

The primary minerals were identified and examined in thin sections by optical microscopy and electron probe microanalysis (EPMA). The EPMA work was carried out on a Cameca SX50 which includes wavelength dispersive spectrometers (WDS) and an energy dispersive spectrometer (EDS). A JEOL JSM-6330F scanning electron

microscope (SEM) was used for high resolution imaging (< 20 µm). Operating conditions for the SEM were 15 kV and with a 12 nA beam current.

Rutile, zircon, and phosphates (e.g. monazite, apatite, and florencite) were identified by EPMA. The whole surface of the thin section, TL-03-86B-5, was examined by backscattered electron (BSE) images so that each grain of rutile, zircon, phosphate, or any other minerals of interest could be detected. Qualitative element concentration analyses was done by EDS, and quantitative elemental analysis of minerals was done by

WDS, using the method described by Stormer et al. (1993) and Scherrer et al. (2000).

Materials used for EPMA standardization: CePO4, LaPO4, SmPO4, NdPO4, ThO2,

GdPO4, and PrPO4, are from the Smithsonian Institute, Washington (Jarosewich et al.,

1991). Analytical conditions for monazite were: 20 kV accelerating voltage; 50 nA electron current, and the electron beam was opened to 5 µm diameter. Analytical conditions for apatite were an accelerating voltage of 15 kV, 15 nA electron current, and the electron beam was opened to 5 µm diameter. Instrument conditions for florencite were an accelerating voltage of 15 kV, a beam current of 20 nA, and a spot size of 1 µm.

Element maps of florencite, apatite, and monazite were constructed on the

Cameca SX50 using MaxView software. Ca, P, and Al maps were produced. ImageJ was used to combine element maps to determine mineral locations within and around the florencite grain and symplectite.

2.2 LA-ICP-MS

In situ REE concentration and U-Th-Pb isotope analysis of florencite, monazite, and apatite was performed using a Varian 810 quadrupole mass spectrometer with the

PhotonMachine Analyte.193 ArF excimer laser ablation system at the University of

Houston. The laser ablation spot size for each mineral was 25 µm and a laser fluence of

4 J/cm2 was used. Analysis time for each sample was 60 seconds that included a 20 s background measurement, 15 s mineral ablation during which the sample signal was measured, and 25 s for complete washout. A He-carrier gas was used to carry the analyte into the mass spectrometer at a rate of 0.550 L/min. An alternating sample-standard bracketing approach was used to correct for instrumental isotopic and instrumental drift where standards were analyzed between every 5 analyses of samples. All sample minerals, with the exception of florencite, were matrix-matched to the standards (i.e. monazite samples were analyzed with monazite standards and apatite samples were analyzed with apatite standards).

Apatite standards used were Bear Lake and Yates Mine, 958 ± 13 Ma and 956-

996 Ma, respectively (Cosca et al., 1995; Thomson et al., 2012). Monazite standards used were the 1099.1 ± 0.5 Ma FC5z standard from the Duluth Complex (Paces and Miller,

1993) as the primary standard and the 45.3 ± 1.4 Ma 554 monazite (Harrison et al., 1999) as an external standard. All age data are reported at the 2 SD level. The Th-Pb age for the 554 standard analyzed in this study is 44.3 ± 0.9, identical within error to the reported

ID-TIMS age. During laser ablation analysis, the following isotopes were measured:

139La, 140Ce, 141 Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb,

175Lu, 201Hg, 202Hg, 204Hg+Pb, 206Pb, 207Pb, 208Pb, 232Th, and 238U.

Trace element data were reduced using Glitter software. U-Th-Pb isotope data were reduced with the Iolite plug-in for Igor (Paton et al., 2011). A second step of data reduction involving common Pb corrections was done in an excel spreadsheet following methods outlined in Shaulis et al. (2010). Isochron and concordia diagrams was constructed with IsoPlot (Ludwig, 1991; Ludwig 2003).

3 Results

3.1 Petrography

Six whiteschist samples were taken from the coesite-bearing pyrope garnet locality near Parigi, Italy. Whiteschists, also known as a kyanite - jadeite - phengite - pyrope - quartz (coesite) granofels, are found in the monometamorphic complex in the

Brossasco-Isasca unit within the Dora Maira (Compagnoni and Rolfo, 1991). The whiteschist consists of pyrope in a quartz matrix with kyanite, minor amounts of talc, +/- ellenbergite, +/- chlorite and accessory minerals including apatite, monazite, rutile, and zircon (Figure 6). One florencite grain was found in the matrix of thin section TL-03-

86B-5, and is surrounded by a symplectite made up of apatite, monazite, and trolleite.

Phengite and phlogopite are present throughout the sample. The coarse phengites display little to no deformation. Unlike other samples within the Dora Maira, chlorite and ellenbergite is not seen in any of our samples.

Figure 6: A.) Sample TL-03-86B-4. Photomicrograph of general matrix composition. Phengite, white mica, has small inclusion of rutile, quartz, and kyanite. B. Sample TL- 03-86B-5. Qtz=Quartz; Ph=Phengite; Rt=Rutile.

3.1.1 UHP Phase Assemblage

The Dora Maira has undergone UHP metamorphism as evidenced by the presence of coesite. Coesite, kyanite, pyrope, and phengite represent the UHP mineral assemblage. Three rock types are present within the coesite-bearing quartzite lens. One is a fine-grained pyrope quartzite, with small pyrope garnets around 1 cm. Another variation is a coarse-grained pyrope quartzite containing large, 25 cm, euhedral or anhedral pyrope garnets. The third rock type is a garnet jadeite quartzite that forms bands within the lens, approximately 5 cm thick (Schertl et al., 1991). The samples studied here are from the fine-grained pyrope quartzite; pyrope size ranges from 0.10 mm to 2 mm and are pale pink. Pyrope and phengite porphyroblasts are surrounded by a fine-grained quartz (formerly coesite)-rich matrix.

Within the garnet, mica, and the matrix are small inclusions of coesite, quartz, zircon, apatite, monazite, rutile, and kyanite (Figure 7). The garnets contain coesite inclusions, which have partially inverted to quartz around the rim (Figure 8). Coesite can be identified optically as having higher relief than quartz and by its bi-axial optical nature. Radial cracking is present around the coesite inclusion in the host garnet, indicating the coesite inclusions underwent a positive volume change duringt ransformation to quartz. Radial cracking is also present around the zircon, phosphates, and rutile inclusions within the garnet. The fractures are filled with very fine-grained

(>10 µm) pyrope garnet, with little variation in composition from the host pyrope garnet.

Small amounts of talc are seen in the garnet fractures. The inclusions in the garnet have

no preferred orientation and are commonly non-idioblastic and range between 10 and 100 microns in size.

Figure 7: A. Plane light photomicrographs of inclusions within pyrope garnet. Quart, kyanite, and rutile grains are inclusions. B. Same as panel A, but with crossed polars. Qtz=Quartz; Gt=Garnet; Ky=Kyanite; Tlc=Talc

Figure 8: Photomicrographs of coesite as an inclusion in garnet, from sample 4. A) Plain light relict coesite (high relief) that is partially inverted to quartz (low relief). Quartz and kyanite grains within garnet. The inclusions do not have a preferred orientation. Radial fractures are around the inclusions, indicating a volume change during exhumation. B. Same as panel A, but with crossed polars. Qtz=Quartz; Cs=Coesite; Gt=Garnet; Ky=Kyanite

3.1.2 Retrograde Phase Assemblage

Retrograde minerals that can be seen within the Dora Maira samples are talc, second-generation kyanite, phlogopite, and quartz. Areas of contact between garnet and white mica reacted to form phlogopite, kyanite, and talc (Figure 9). The rim forming around the garnets is associated with the reaction phengite + pyrope + H2O = phlogopite

+ kyanite + talc (Chopin, 1984). Phlogopite within the reaction rims around garnet is likely related to the previous reaction and also forms with the reaction: pyrope + phengite

= phlogopite + kyanite + quartz (Chopin, 1984). Elongated quartz grains are still preserved in the matrix, and developed radially to any interface with other minerals present (Figure 10). The quartz matrix is composed of recrystallized polygonal grains that display undulatory extinction.

Figure 9: Talc rim forming around garnet. A-D.) Sample 5; E-F) Sample 7. Qtz=Quartz; G=Garnet; Ky=Kyanite; Tlc=Talc; Ph=phengite.

Figure 10: A-B.) Sample TL-03-86B-4. C-D). Sample TL-03-86B-5. Elongated quartz grains, parallel to one another. Qtz=Quartz; Ph=Phengite; Gt=Garnet.

3.1.3 Minor Phase Assemblage

Minor minerals within the Dora Maira are rutile, zircon, and various phosphates.

All minerals are present as inclusions within the pyrope garnet and in the matrix with no preferred orientation and spatial relation. Zircon grains are less than 25 microns, while rutile grains range from 50 microns to 150 microns.

Phosphates within the samples consist of unaltered monazite and a monazite apatite symplectite. Unaltered monazite is present within each of the samples, in both the matrix and as inclusions within garnet. Apatite is only present as inclusions in garnet and 24

matrix that contain monazite and displays a symplectite texture (Figure 11). Three distinct textures are displayed by the phosphates: 1) A coarser grained apatite and monazite symplectite (Figure 11 c-d), 2) a finer grainer apatite and monazite (Figure 11 a), 3) an unaltered monazite (Figure 11 b,e,f) . The darker zones on BSE images indicated apatite, while lighter zones indicated monazite. The coarse-grained apatite and monazite symplectite only occur in the quartz and phengite matrix, while one finer grained apatite and monazite symplectite occurs as an inclusion in the pyrope garnet,

(Figure 11 a). Unaltered monazite grains are generally within the garnet as inclusions, with a couple unaltered monazite grains in the quartz and phengite matrix. The symplectite and unaltered monazite display no preferred orientation and have no spatial relation. Monazite inclusions in the garnet mainly occur within the garnet fractures,

(Figure 11 f).

Figure 11: BSE images of phosphate grains. Apatite, the darker mineral, and monazite, lighter mineral. A-B.) TL-03-86B-4. C-D.) TL-03-86B-5. E-F.) TL-03-86B-7. Ap=Apatite; Mnz=Monazite; Gt=Garnet; Ph=Phengite; Qtz=Quartz

3.1.4 Florencite

Florencite (Ce, La)Al3(PO4)2(OH)6, is found in the matrix of sample TL-03-86B-

5. The florencite grain is 100 µm in diameter and displays high relief. In plain light, the florencite grain appears colorless to pale yellow (Figure 12). There is a large symplectite rim that surrounds a smaller grained symplectite rim (Figure 13b). BSE images show the coarse-grained and fine-grained symplectite rim (Figure 13 a&b). The large symplectite rim is composed of 10 to 30 µm grains and the finer-grained rim is composed of > 5µm grains. Each rim consists of apatite, monazite, and trace amounts of trolleite, an aluminum phosphate Al4(PO4)3(OH)3. In Figure 13b, a rutile grain occurs as an inclusion within the florencite grain; corundum occurs on both sides of the rutile grain. A monazite and apatite symplectite also appears as an inclusion within the florencite grain, adjacent to the rutile. During EPMA analysis, florencite became unstable and underwent a dehydration reaction producing a very fine-grained symplectite-like texture of unidentified minerals.

The florencite grain and apatite + monazite symplectite rim is surrounded by fine- grained kyanite. Figure 14 displays element maps of the grain, highlighting Ca, P, and

Al; while Figure 15 merges each element map together to get a spatial relation of each mineral. Light blue represents apatite, light green represents florencite, dark green represents monazite, and red represents kyanite around the grain and corundum in the left-hand side of the florencite grain. Trolleite is either the pink color spread throughout the sample or the red within the symplectite surrounded by apatite and monazite.

Reactions between the florencite, apatite and monazite symplectite, and kyanite are further explained in the discussion.

Figure 12: Microphotographs of florencite from TL-03-86B-5. A. Photomicrograph of florencite. B. Same as panel A, but in cross polars. C-D. Backscatter electron images of florencite, displaying a symplectitic rim of apatite and monazite, within a whiteschist from the Dora Maira. The florencite grain has rutile inclusion. Electron microprobe analyses are listed in Table 6. Ap=Apatite; Mnz=Monazite; Fl=Florencite; Cr= Corundum; Rt=Rutile; Ky=Kyanite; Qtz= Quartz

Figure 13: Backscattered electron photographs of Florencite. Ap=Apatite; Mnz=Monazite; Fl=Florencite; Cr= Corundum; Rt=Rutile; Ky=Kyanite; Tr=Trolleite

Figure 14: Element Maps of Florencite. Element distribution patterns of florencite, apatite, and monazite showing the varying Ca, Al, P, and Ce concentration. The scale of shadings on the right-hand side of the pictures indicates the counting rates per time unit. A.) Al Concentration. High Al concentrations around the grain are kyanite, with small amounts of kyanite in the symplectitic rim. High Al surrounding rutile represents corundum. B.) P Concentration. C.) Ca concentration. Apatite grains have a higher Ca content than monazite, highlighting the apatite in the symplectic rim

Figure 15: Combined element maps of florencite, highlighting Ca, P, and Al. Light blue represents apatite, light green is florencite, dark green is monazite, and red is kyanite around the grain and corundum in the left-hand side of the florencite grain. Trolleite is either the pink color spread throughout the sample or the red within the symplectite surrounded by apatite and monazite.

3.2 Trace Element Compositions

3.2.1 Phosphates

Electron microprobe data for monazite, taken as a single spot within monazite,

(Table 1) reveal a characteristic strong LREE enrichment, especially in La2O3 (up to

21.78 wt.%) and Ce2O3 (up to 33.51 wt.%). Other REE are present at lesser concentrations: Pr2O3 ranges from up to 7.29 wt.%, Nd2O3 ranges up to 7.87 wt.% with an average of 6.82 wt.%, Gd2O3 up to 0.43 wt.%, and ThO2 up to 4.48 wt.%. LA-ICP-

MS trace element data for monazites, Table 2, show an enrichment in LREE with La2O3

and Ce2O3, up to 17.75 wt.% and 28.61 wt.%, respectively. Table 3, displays the REE concentrations in ppm.

Electron microprobe data for apatite (Table 4) demonstrate LREE enrichment, especially in La2O3 (up to 25.17 wt.%) and Ce2O3 (up to 35.48 wt.%). Other REE are present at lower concentrations: Pr2O3 ranges from 0.3 to 2.1 wt.%, Nd2O3 ranges from 1 to 5.5 wt.%, Gd2O3 between 0.01 to 0.71 wt.% and ThO2 from up to 4.48 wt%. Table 4 gives all the microprobe analyses. Apatite is only present with monazite as inclusions in garnet and the matrix. The apatite and monazite grains display a symplectite texture.

Electron microprobe analyses of apatite are mixed, due to the close proximity to monazite.

Electron microprobe data for florencite are presented in table 5. The florencite mineral group contains three named minerals: La-florencite, Ce-florencite, and Nd-

Florencite. The florencite within the sample is 54% Ce-Florencite, 34.5% La-Florencite, and 11.5% Nd-Florencite. Figure 16 displays REE concentrations of monazite, florencite, and monazite-apatite symplectite.

Table 1: Electron probe microanalytical data for Monazite from sample TL-03-86B-5. B.d= below detection. Atom per formula unit based on 4 oxygen. * = Detection Limits 0.02

Table 2: Average LA-ICP-MS trace element concentrations in wt% for phosphates from sample Tl-03-86B-7 and TL-03-86B-5. B.d.=below detection

Table 3: Average LA-ICP-MS trace element concentrations in ppm for phosphates from sample Tl-03-86B-7 and TL-03-86B-5

Figure 16: Chondrite-normalized rare earth element concentrations of monazite, florencite, and monazite-apatite symplectite in Sample TL-03-86B-7. Blue lines indicate unaltered monazite; red lines represent apatite and monazite symplectite; green represents florencite.

Table 4: Electron probe microanalytical data for Apatite from sample TL-03-86B-5. B.d= below detection; na=not analyzed; * = Detection Limits 0.02.

Table 5: Electron probe microanalytical data in weight percent for Florencite from sample TL-03-86B-5. B.d= below detection; Atoms per formula unit is based on 11 oxygen;.*= Detection Limits 0.02

3.2.2 Trace Elements in Garnet

Garnet trace element concentrations, Figure 17 a-b, were taken from rim to rim from sample TL-03-86B-8. There is a strong enrichment of MREE and HREE over

LREE in the pyrope.

Figure 17 A: Pyrope Garnet Trace Element Concentrations. B. Garnet Trace Element Concentrations chondrite normalised

3.3 Ages

Unaltered monazite 238U/206Pb and 232Th/208Pb isochron ages are 31.48 ±0.22 Ma and 32.67 ±0.70 Ma, respectively (Figure 18). Symplectite monazite, apatite, and florencite 238U/206Pb and 232Th/208Pb isochron ages are 31.3 ± 1.7 Ma and 1.87 ± 4.3 Ma, respectively (Figure 19). A monazite 207Pb/235U Concordia age determined for common-

Pb corrected data is 31.73 ± 0.85 Ma (Figure 20). Isotope data for each sample are listed in Table 6. All monazite analyses contained substantial amount of non-radiogenic lead, often greater than 10 ppm precluding common-Pb corrections for most analyses.

Figure 18: Isochron diagrams for unaltered monazites from sample TL-03-86B-7 and TL- 03-86B-5.

Figure 19: Isochron diagrams for altered monazite, florencite, and apatite from samples TL-03-86B-7 and TL-03-86B-5.

Figure 20: Concordia diagram

4 Discussion Geochemical analysis of garnet, apatite, monazite, florencite, and additional matrix minerals is pertinent to understanding the metamorphic history of the Dora Maira.

The ages and petrological data of the whiteschists from this study agree with many previous works (Chopin, 1984; Tilton et al., 1989; Tilton et al., 1991; Duchene et al.,

1997; Vaggelli, et al., 2006).

4.1 Phosphates Phosphates present within the Dora Maira whiteschists are florencite, unaltered monazite, and a monazite apatite symplectite.

4.1.1 Florencite

This is the first evidence of florencite being found in the Brossasco-Isasca Unit of the Dora Maira nappe. Florencite has been found in the Gran Paradiso, one of the three

Inner Penninic nappes in the western Alps (Radulescu et al., 2009). In the high pressure micaschist unit of the Gran Paradiso, florencite inclusions are found within monazite; suggesting that florencite is a prograde mineral (Radulescu et al., 2009). Nagy et al.

(2002) found florencite and monazite inclusions in kyanite quartzites and leucophyllites in the Sioron Hills, eastern Alps. This area, much like the Gran Paradiso and Dora Maira, experienced Alpine metamorphism in the Eocene, but did not undergo ultra-high pressure metamorphism.

Florencite was likely formed by fluids during Mg-metasomatism that formed the whiteschists in the Dora Maira. Florencite is stable at pressures and temperatures of 1.5

GPa at 500° C to 2.5 GPa and 700° C (Barthelemy and Chopin, 2006; Radulescu et al.,

2009). Thus, it likely remained stable from low P-T to UHP metamorphic conditions.

During retrograde metamorphic conditions, it is likely that infiltrating fluids facilitated the formation of the monazite and apatite symplectite rim.

The florencite grain and symplectic rim of apatite and monazite is surrounded by kyanite. Florencite REE data (Figure 16) show a strong enrichment in LREE compared to HREE; indicating a preferential partitioning of LREE into florencite rather than

HREE. Florencite also displays a negative Eu anomaly that may be a result of fluid-rock interaction during metamorphism (Nagy et al. 2002). Fractionation of REE can be seen between florencite and monazite inclusions (Nagy et al. 2002). As florencite prefers

LREE, monazite inclusions are depleted in LREE when in contact with florencite (Cook et al., 2013).

4.1.2 Monazite Apatite Symplectite

The monazite apatite symplectite are interpreted to form around florencite during retrograde metamorphism due to fluid infiltration. Scherrer et al. (2001), found evidence of a similar fine-grained monazite, apatite, and corundum rim surrounding bearthite,

Ca2Al(PO4)2(OH), in the UHP unit of the Dora Maira. Individual monazite and apatite symplectite grains within our samples (Figure 11 c-d) may have formed around florencite

or bearthite during retrograde metamorphism due to fluid interaction. Smaller bearthite or florencite grains would completely retrogress to apatite and monazite, while larger grains would just form a rim of apatite and monazite during decompression.

Fluid present during prograde and peak metamorphism, from fluid inclusion studies done by Ferrando et al., (2009), contain Na, Cl, Mg, Al, and Si. However prograde fluids were rich in MgCl2; while peak metamorphic fluids were rich in Si, Al, and contained high amounts of Ca. Large amounts of Ca may help form apatite.

However, apatite displays little to no OH. Further work needs to be done to better characterize and understand its formation and occurrence.

4.1.3 Ages

U-Pb and Th-Pb isochron ages of the monazite are 31.48 ± 0.22 and 32.67 ± 0.70

Ma, respectively (Figure 18). The 206Pb/204Pb ratios for the monazite ranged from 19.0 –

360 and each monazite contained > 10 ppm common Pb. Monazite from two samples that had a 206Pb/204Pb concentration greater than 100 produced a U-Pb Concordia age of

31.73 ± 0.85 Ma (Figure 19). The symplectites (monazite, apatite, plus florencite) yielded 238U/204Pb and 232Th/204Pb isochron ages of 31.3 ± 1.7 and 31.8 ± 4.3 Ma, respectively (Figure 18), and are indistinguishable in age and initial Pb isotope composition from the unaltered monazites.

Normally, monazite does not incorporate significant amounts of common Pb into its structure during metamorphism, meaning all of its Pb is radiogenic from decaying Th

and U. Common Pb in my monazite analyses is evidenced by low 206Pb/204Pb and

208Pb/204Pb ratios (~19 and 40, respectively). It is difficult to incorporate common Pb into the monazite structure, yet common Pb has been measured in many studies of natural monazite. Monazites formed from ore or hydrothermal deposits display a higher amount of non-radiogenic or common lead. Previous studies: Hoisch et al. (2008); Janots et al.

(2012); Catlos (2013), have suggested that the common Pb is derived from grain boundaries or along microfractures in the grain, rather than being in the monazite grain.

In the Dora Maira, a high concentration of common Pb was found in all monazites; the high 206Pb/204Pb and 208Pb/204Pb ratios observed in some grains is due to the greater relative concentration of U and Th, respectively (Table 6). Hydrothermal fluids may be a reason for the high concentration of common Pb found in monazite, caused by dissolution-reprecipitation reactions (Catlos, 2013).

4.2 Implications of REE Concentrations

Trace element compositions from this study and previous studies (Paquette et al.,

1989; Ferrando et al., 2009; Grevel et al., 2009; Radulescu et al., 2009) show variations among samples. This may be due to varying chemistry in metasomatic processes. The

LREE enrichment and Eu anomalies of the monazites can be explained by the Hercynian granitoid protolith.

During pyrope garnet formation, HREE accumulated in the garnet (Grevel et al.,

2009). Some unaltered monazite equilibrated with the garnet during this time and

incorporates smaller amounts of HREE into its structure. The monazite apatite symplectite display lower amounts of HREE compared to the equilibrated monazite.

During retrograde metamorphism and associated garnet breakdown, HREE become mobilized and were incorporated into the monazite apatite symplectites and some unaltered monazite. This HREE enrichment in monazite associated with the breakdown of garnet is dated at 31.48 ±0.22 Ma.

The phosphates record and incorporate REE liberated by garnet break-down likely associated with the reaction phengite + pyrope + H2O = phlogopite + kyanite + talc

(Chopin, 1984). The upper stability of talc only occurs at high temperatures and pressures within the coesite-quartz stability field (Chopin, 1984; Schertl et al., 1991;

Simon et al., 1997). The coexistence of talc-phengite and pyrope-talc with coesite are high pressure assemblages (Chopin, 1984). The pyrope-coesite-talc-kyanite assemblage implies the Dora Maira experienced temperatures between about 700 and 800 °C at pressures of 28 kbar (Chopin, 1984). Phosphate ages from this study, 31.48 ± 0.22 Ma, likely reflect the timing of this process. The pressure and temperature conditions of this reaction could lie above the quartz-to-coesite transition (e.g. Chopin, 1984; Schertl et al.,

1991), so the monazite ages may record UHP conditions during the retrograde path. My samples may not have experienced the lower pressure garnet break down reaction pyrope

+ H2O = chlorite + kyanite + talc, but I do not see appreciable amounts of chlorite within my samples (HP retrograde path of Schertl et al., 1991). Phlogopite within the reaction rims around garnet is likely related to the reaction: pyrope + phengite = phlogopite +

kyanite + quartz (Chopin, 1984). Areas of contact between garnet and white mica reacted to form phlogopite, kyanite, and talc.

4.3 P-T-t Path

4.3.1 UHP Terranes

The P-T-t path (Figure 21) indicates that the Dora Maira experienced UHP metamorphism with a steep P-T path that displays a rapid increase in temperature and pressure during the peak metamorphic conditions. Unlike many other UHP terranes, the

Dora Maira did not experience an increase in temperature during exhumation. The retrograde P-T path is unlike many other UHP terranes in that it is similar to the inferred prograde path (i.e. along a steep P-T gradient). Lago di Cignana, western Alps, has a similar P-T path to the Dora Maira (a steep prograde and retrograde path) but it experienced lower peak temperatures (Reinecke, 1991; McClelland and Lapen., 2013 for a summary). Exhumation rates within the Lago di Cignana are estimated at 1-3 cm/yr

(McClelland and Lapen, 2013). The Dabie-Sulu UHP terrane in China, experienced a prolonged UHP path and slow exhumation rates (0.5 cm/yr) evidenced by varying zircon ages. UHP terranes in northeast Greenland experienced peak metamorphic conditions,

3.6 GPa and 950 °C, for 10 to 15 My (McClelland and Lapen, 2013). Exhumations rates were calculated at 4 mm/yr. Terranes that experienced UHP metamorphism have different P-T paths, but many have a prolonged history at UHP conditions or have an increase in temperature during decompression. However, the Dora Maira and Lago Di

Cignana, both in the western Alps, had a steep P-T path and rapid exhumation.

4.3.2 Monazite

The monazite ages reflects a cooling age or retrograde metamorphism age for the

Dora Maira, while garnet reflects the age of peak metamorphic conditions at UHP. In order to constrain the monazite on the Dora Maira P-T-t path, the closure temperature of monazite (720-750° C) defines the lower limit (Zhou and Hensen, 1995). The closure temperature of the Sm–Nd system in garnet remains controversial, ranging from 600 °C to 1050 °C. Zhou and Hensen (1995) suggest a Sm-Nd closure temperature in garnet of

>700 °C. The Lu-Hf system is expected to have similar closure temperatures to Sm-Nd.

Ages of phosphates (e.g. monazite, apatite, and florencite) reflect the cooling and decompression of the Dora Maira.

4.3.3 Exhumation Rates

The monazite ages of this study are younger than ages of peak metamorphic conditions as shown by the Lu-Hf garnet-matrix age of 34.1 ± 1.0 (Duchene et al., 1997, recalculated with the 176Lu decay constant of Scherer et al., 2001) and the 35.1 ± 0.9 Ma

U-Pb age of UHP titanite (Rubatto and Hermann, 2001). The monazite ages are consistent with, but slightly younger, than U-Th-Pb ages of monazite by Tilton et al.

(1991). U-Pb ages of retrograde titanite, 32.9 ± 0.9 Ma and 31.8 ± 0.5 Ma (Rubatto and

Hermann, 2001), plot significantly lower on the P-T-t path than the retrograde monazites from this study. However, they are within error of our 31.48 ± 0.22 Ma age.

Exhumation rates can still be calculated using 31.70 Ma for the monazites and 32 Ma and

31.3 Ma for retrograde titanite ages. If the monazite ages record the UHP retrograde path, initial exhumation rates of the Dora Maira nappe are significantly greater than the exhumation rate of 3.4 cm/yr previously proposed by Rubatto and Hermann (2001). The

Dora Maira did not have a constant exhumation rate; using UHP ages, retrograde phosphate and titanite ages, a more accurate determination of exhumation rates can be achieved. Calculated exhumation rates from this study indicate the Dora Maira began at a rate between 0.42 and 1.13 cm/yr from 120 km to 105-85 km, between a UHP garnet age of 34.1 ± 1 Ma and a retrograde monazite age of 31.48 ± 0.22 Ma. Exhumation rates increased to 15 to 21.25 cm/yr from 105-85 km (31.48 ± 0.22 Ma) to 40-20 km (32.9 ±

0.9 Ma; 31.8 ± 0.5 Ma). This shows that the Dora Maira had varying exhumation rates during its history and they are faster than previously proposed.

Figure 21: Pressure-Temperature diagram for the Dora Maira. DM= Dora Maira, Py=Pyrope, Cs=Coesite, Phe=Phengite, Tc=Talc, Ky=Kyanite, Chl=Chlorite, Phl=Phlogopite, Q=Quartz . Reaction 1:Pyrope + Coesite + H2O = Talc + Kyanite; Reaction 2: Pyrope + Phengite + H2O = Phlogopite + Kyanite + Quartz; Reaction 3: Pyrope + H2O = Chlorite + Talc + Kyanite (Modified from McClelland and Lapen, 2013)

Conclusions

Understanding the rates of subduction, burial, and exhumation requires accurate and precise age determinations of various stages along a unit’s pressure-temperature path; this is determined by linking measured ages to the metamorphic conditions at that age.

Ultra high pressure metamorphism occurred in the Dora Maira at 34.1 ± 1 Ma (Duchene et al., 1997) and monazites constrain the retrograde path at 31.48 ± 0.22 Ma (this study).

Calculated exhumation rates from this study indicate the Dora Maira began at a rate between 0.42 and 1.13 cm/yr from 120 km to 105-85 km; but began to quicken to 15 to

21.25 cm/yr during retrograde from 105-85 km to 40 km (32.9 ± 0.9 Ma) and 20 km

(31.8 ± 0.5 Ma). This shows that the Dora Maira experienced varying exhumation rates during its history and that prove exhumation rates were considerably faster than previously proposed (3.4 cm/yr, Rubatto and Hermann, 2001). Finally, this is the first study that has found florencite within the Dora Maira Nappe. Florencite is interpreted to have formed either at peak metamorphic conditions or during prograde metamorphism, with monazite and apatite forming a rim during retrograde metamorphism and exhumation.

Project 2: Garnet Geochronology

1 Introduction

Garnet geochronology is used to determine peak metamorphism, involving the

Sm-Nd and Lu-Hf isotope systems. Garnet is commonly zoned in trace elements, and these zones may represent changes in P-T-t conditions or changes in trace element availability during garnet growth.

2 Methods

2.1 Trace Element

In situ analysis of pyrope garnet was performed using the Varian 810 quadrupole mass spectrometer with the PhotonMachine Analyte.193 ArF excimer laser ablation system, at the University of Houston. The laser ablation spot size for each mineral was

25 µm and a laser fluence of 4 J/cm2 was used. Analysis time for each sample was 60 seconds; 20 seconds during background, 15 seconds during the laser ablation, and 25 seconds for washout. The gas carrier used was helium that had a flow rate of 0.550

L/min. REE concentrations taken from core to rim from garnet, within sample TL-03-

86B-8. Trace element data were reduced using Glitter software; while additional data reduction was done in an excel spreadsheet following methods outlined in Shaulis et al.

(2010).

2.2 Sm-Nd and Lu-Hf

Whole garnet and whole rock samples were crushed and sieved to collect pyrope garnets. White mica was separated from the samples. Heavy liquid separation was done using acetylene tetrabromide (density = 2.96 g/cc), to separate the garnet (3.74 g/cc) from quartz (2.65 g/cc). Each sample was washed 10 times with acetone after separation.

Hand picking was done under an optical microscope to remove any garnet fragments with inclusions and other heavy minerals. The garnet samples were crushed to 325 um using an agate mortar and pestle.

Phosphate, zircon, rutile, and other inclusions are present throughout the garnets.

Multiple rounds of leaching were done to rid the garnets of inclusions follow the procedure in Table 7. Leaching techniques were modified from Amato et al. (1999) and

Zhou and Hensen (1995). After leaching the whole garnet and whole rock samples were combined for a total weight of 0.11558 g for Sm-Nd analysis. Nd concentrations in garnet was assumed at 100 ppb for spike calibration and whole rock Nd concentrations was 24.475 ppm, from Tilton et al. (1991), Mica Nd concentrations are assumed to be half of that of the whole rock, at 12.2375 ppm.

Table 7: Leaching Methods

Dissolution of garnet (combined whole garnet and whole rock garnet), mica, whole rock without garnet, and whole rock with garnet was done with 4 ml 29 M HF, 0.5 ml 15 M HNO3 at 110 ºC for 3 days. After drying down, fresh HF and HNO3 was added at 110 ºC for 3 days. This was followed by 2 ml 15 M HNO3, 2 ml H2O at 110 ºC for 24 hours. Precipitated fluoride salts are dissolved using 3 ml 6 M HCl at 110 ºC for 24 hours, repeatedly, until the whole sample was dissolved. Additional dissolution was needed after 1 week; 4 ml 29 M HF, 0.5 ml HNO3 were added for 3 days at 110 ºC, followed by 3 ml 6 M HCl at 110 ºC for 24 hours until sample was fully dissolved.

First, the bulk suite of rare earth elements was isolated in RE Spec cation exchange column chemistry, and then Sm-Nd was isolated through HIBA column chemistry. Data analysis was performed in the MC-ICP-MS Laboratory at University of

Houston using a Nu Instruments Plasma II - Multi Collector Inductively Coupled Plasma

Mass Spectrometer.

Table 8: Sm and Nd element and isotope data for the Dora Maira pyrope garnet

3 Results

3.1 Ages

Pyrope garnet and whole rock analyses yields Sm/Nd ratios of 2.4422 and 2.0736, respectively (Table 8). The initial 143Nd/144Nd ratio is 0.51223 ± 0.00016 with a MSWD of 9.7. Sm-Nd analysis from garnets, whole rock, whole rock without garnet, and mica all have a low Sm-Nd ratio (Table 8).

The Sm-Nd age from garnets obtained from this study was -8 ± 180 Ma (Figure

23) and the garnet Lu-Hf age is 35.74±0.71 Ma (Figure 24) with an initial 176Hf/177Hf ratio of 0.282 and a MSWD of 1.7 (Lapen, 2007, unpublished data).

3.2 REE Concentrations

REE compositions of Lu-Hf and Sm-Nd were taken from rim to rim of a garnet,

(Figure 22). Lu-Hf ratios increase towards the center of the garnet grain. Sm and Nd concentrations decrease but increase towards the rims. The Sm-Nd ratio in the garnet ranges from 4-5 ppm.

Figure 22: Sm-Nd isochron.

Figure 23: Lu-Hf isochron from garnets (Lapen, 2007, unpublished data).

Figure 24: REE concentrations of Lu-Hf and Sm-Nd in pyrope garnet. Taken from rim to rim (Lapen, 2007, unpublished data).

4 Discussion

4.1 Sm-Nd issues

Garnet geochronology is used to determine peak metamorphism, involving the

Sm-Nd and Lu-Hf isotope systems. These systems require analyzed garnet to be inclusion free (or leached to remove inclusions), Sm-Nd and Lu-Hf must have remained in a closed system since the formation of the high pressure or ultra-high pressure mineral assemblage, and must not contain any older inherited components (Lapen et al., 2003).

Mineral inclusions in garnet affect the calculated ages, and in turn estimated of P–T–t paths and garnet closure temperatures. The closure temperature of inclusions in the garnet, the thermal history of whole rock, and the ages of the garnet and the inclusions all factor into the effect inclusions have on dating garnet (Scherer et al., 2000). Previous work shows that accessory mineral inclusions (e.g. monazite, zircon, apatite) within garnet make it difficult to obtain an accurate age (Amato et al., 1991; Zhou and Hensen,

1995; Vance et al., 1998; Prince et al., 2000; Scherer et al., 2000; Lapen et al., 2003;

Kohn et al., 2008). Acid leaching is used to rid the garnets of inclusions.

These inclusions have affected the ages calculated from the pyrope garnets. If these inclusions reach isotopic equilibrium with the garnet at time of growth, then the inclusion age with be the same as the garnet. If the inclusion does not reach equilibrium it results in an erroneous age that is different from the true garnet age (Prince et al.,

2000). Mineral inclusions that are LREE-enriched have low Sm-Nd ratios, causing the measured Sm-Nd ratios to less than that of the garnet (Lapen et al., 2003). Sm-Nd garnet geochronology is greatly affected by the presence of phosphate inclusions which lower

precision and accuracy while dating (Chopin et al., 1999; Anczkiewicz and Thirlwall,

2003). The precision of Sm-Nd dating is also limited because of the small Sm-Nd ratio in garnets (Duchene et al., 1997).

The Sm-Nd garnet whole rock age obtained from this study was -8 ± 180 Ma

(Figure 23). Inclusions that are low in Sm-Nd but have a high Nd concentration seem to dominate the Sm-Nd ratio of mineral separates (Cliff et al., 1998). While garnets were handpicked and leached to try to dissolve all inclusions; inclusions must have been present to produce such low Sm-Nd ratios and age. The garnet Lu-Hf age of 35.74 ± 0.71

Ma is interpreted as the age of peak metamorphism (Lapen, 2007, unpublished data).

5 Conclusion

An accurate age of peak metamorphism in the Dora Maira was achieved with Lu-

Hf garnet geochronology. Garnet Sm-Nd geochronology was not able to produce a usable age because of the inclusions that were present within the garnet. These inclusions have affected the calculated ages and cause the measured Sm-Nd ratios to less than that of the garnet (Lapen et al., 2003). Since phosphate inclusions are prevalent throughout the garnet, a better method of dating the whiteschists of the Dora Maira would be U-Pb phosphate geochronology.

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