Heat sources for Variscan high-temperature–low-pressure metamorphism: Petrochronological constraints from the Trois Seigneurs massif, French Pyrenees

Abstract

High-temperature–low-pressure metamorphism is commonly associated with intermediate to felsic magmatism in continental orogenic belts. The heat budgets and transfer mechanisms responsible for such elevated temperatures and partial melting of the upper crust are uncertain. The Trois Seigneurs massif, French Pyrenees, preserves a structurally continuous record of Variscan high-temperature–low-pressure metamorphism through a sequence of upper-to-mid-crustal Paleozoic metasedimentary rocks. Conventional thermobarometry and phase equilibria calculations show that metamorphic conditions span ~2.5 kbar, 575°C to suprasolidus conditions of ~6 kbar, 700°C. Peak temperatures depend strongly on depth: temperature gradients of 50–60°C/km are present through the uppermost 12 km of the section; deeper portions (12–20 km) define restricted temperature conditions of ~650–700°C. The lowest-grade metamorphic rocks preserve the largest spread in monazite ²⁰⁶Pb*/²³⁸U dates, from c. 325–285 Ma, while the spread in dates is restricted to c. 305–290 Ma in the highest-grade rocks. Within this spread, each sample yields a well-defined population of monazite ²⁰⁶Pb*/²³⁸U dates with peaks at c. 305 Ma in the andalusite schists, 295 Ma in the sillimanite schists, and 300 Ma in the migmatite sample. Monazite trace-element compositions capture a systematic change with decreasing date and increasing metamorphic grade, including a more negative Eu-anomaly and decreasing Sr concentrations, consistent with co-crystallizing feldspar; increasing HREE and Y contents, consistent with xenotime breakdown; and decreasing Th/U, reflecting increasing U content during breakdown of inherited zircon. Zircon rims from a granite unit that formed via partial melting of the Paleozoic sedimentary package yields a ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U concordia age of 304.1 ± 3.73 Ma. These rims have trace-element compositions reflecting cogenetic apatite and zircon growth during granite formation. Zircon from a calc-alkaline granodiorite intrusion preserves a 40 Ma record of melt-related activity in the lower crust that preceded the regional thermal climax. We interpret these petrochronological data to show that the Trois Seigneurs field gradient including andalusite schist and biotite granite samples represents a genuine geotherm through Variscan orogenic crust during the regional thermal climax at 305 Ma. When combined with constraints from other Pyrenean massifs, the form of the geotherm is consistent with a thermal scenario in which heat is advected to the upper crust by intermediate-composition magmas generated in the lower crust. A simple thermal model for this process indicates that anatexis in the upper crust may plausibly occur within 10 Ma of the initiation of the lower-crustal melting. Such a thermal scenario, however, requires focusing of melt through a fertile lower crust and an elevated Moho heat flux. We suggest that this process may have controlled the attainment of high-temperature–low-pressure metamorphic conditions along the Variscan belt and may currently be operating in zones of post-orogenic continental extension.