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Timescales of partial melting in the Himalayan middle crust: insight from the Leo Pargil dome, northwest India

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Abstract

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and post-kinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-m-wide) crosscutting dikes extending at least 100 m. Three-dimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U–Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U–Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U–Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U–Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steady-state equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1–2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.

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Acknowledgments

Funding for this project was provided by National Science Foundation grants (EAR-0911416 and EAR-1119380) awarded to J. Cottle and (EAR-0911561) to M. Jessup. We thank A. Kylander-Clark and G. Seward for assistance with LA-ICPMS and EPMA data collection. P. E. Lee provided valuable assistance in the field. We wish to thank two anonymous reviewers for their thoughtful reviews of an earlier version of this manuscript as well as Franck Poitrasson for his helpful editorial assistance.

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Correspondence to Graham W. Lederer.

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Communicated by F. Poitrasson.

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Appendix 1: Analytical methods

Appendix 1: Analytical methods

EPMA X-ray chemical mapping

Analyses were carried out in beam scan mode using 15 kV accelerating voltage, 200 nA beam current (equating to a ~1 μm3 interaction volume), and a dwell time of 25 ms. Monazite is relatively homogeneous with respect to the LREE, but may show considerable variation in Th, U, and Y, with the geochemical behavior of Y serving as a proxy for HREE (Foster et al. 2004). Understanding the distribution of these elements potentially provides important information on the growth history of individual crystals (Cocherie et al. 1998; Stepanov et al. 2012; Williams et al. 1999, 2007; Zhu and O’Nions 1999).

LA-MC-ICPMS U–Th/Pb geochronology

The laser system utilizes a HelEx (“helium excimer”) sample cell (Eggins et al. 1998, 2005) designed for enhanced signal intensity and rapid washout times using He carrier gas. Helium carrier gas conveys the laser aerosol to a glass mixing-bulb where Ar is added to stabilize the input to the plasma. Boiled liquid argon and ultra-high purity helium are passed through activated charcoal and gold-coated quartz sand filters upstream of the mass flow controllers to reduce 204X (where X includes 204Pb and 204Hg) backgrounds to <200 cps. The collector array on the Nu Plasma is configured to measure 238U and 232Th on two high-mass side Faraday cups equipped with 1011 ohm resistors and 208Pb, 207Pb, 206Pb, and 204X on four low-mass side ETP discrete dynode secondary electron multipliers.

Ablations were conducted for 40 s each at 4.8 J/cm2 fluence, a frequency of 3 Hz, and a pit diameter of approximately 7 μm yielding craters 5–6 μm deep (as assessed by optical microscopy). Five to ten spot analyses were collected on each monazite grain, targeting domains with different trace element chemistry visible in X-ray maps, including cores and rims. Location of individual laser pits was confirmed with an optical microscope after analysis. Utilizing a standard-sample bracketing technique, analyses of reference materials with known isotopic compositions were measured before and after each set of ten unknown analyses. Reference materials consisted of several monazite grains or fragments with matrices similar to the unknowns and published isotopic ages including “44069” (424 Ma Pb/U ID-TIMS age; Aleinikoff et al. 2006), “FC-1” (55.7 Ma Pb/U ID-TIMS age; Horstwood et al. 2003), and “554” (45 Ma Pb/Th age; Harrison et al. 1999). Concordia and weighted mean date plots were calculated in Isoplot v2.4 (Ludwig 2000) using the 238U, 235U, and 232Th decay constants of Steiger and Jäger (1977).

LA-ICPMS Trace Element analysis

Chemical domains identified during X-ray mapping were targeted for trace element analysis by sampling next to U–Th/Pb ablation pits. Following analysis, optical petrographic microscope images of monazite grains were compared to X-ray maps to ensure trace element and U–Th/Pb ablation pitfall within targeted chemical domains. Additionally, the ablation profile was inspected for anomalous or abrupt changes in intensity, and based on this three-dimensional assessment of whether each spot analysis sampled a single domain, analyses located on domain boundaries or fractures were excluded from the data set. Ablations were conducted for 30 s each at 3.2 J/cm2 fluence, a frequency of 3 Hz, and a pit diameter of approximately 6 μm yielding craters 3–4 μm deep.

Because the AttoM is a relatively new instrument, we briefly describe the analytical routine below. In Linked Scan mode, the magnet current is ramped up and down in a controlled manner such that a complete cycle (m/z 6–250–6) can be completed every 220 ms (100 ms each direction + 20 ms settle time). To maximize counting time, each isotope of interest is deflected into the single discrete dynode secondary electron multiplier by simultaneous use of the magnet and the post-ESA deflectors. As the magnet sweeps, each mass remains in the ion counter for up to 40 % of its mass along the magnet sweep. Elemental abundances and their uncertainties are calculated using a simple matrix-matched sample-standard bracketing approach in Iolite v. 2.1.2 (Paton et al. 2010), including corrections for baseline and instrumental drift.

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Lederer, G.W., Cottle, J.M., Jessup, M.J. et al. Timescales of partial melting in the Himalayan middle crust: insight from the Leo Pargil dome, northwest India. Contrib Mineral Petrol 166, 1415–1441 (2013). https://doi.org/10.1007/s00410-013-0935-9

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Keywords

  • Leucogranite
  • Monazite
  • U–Th/Pb geochronology
  • Anatexis
  • Himalaya