Timescales of continental lower crust formation: constraints from garnet Lu-Hf geochronology

Описание: Timescales of continental lower crust formation: constraints from garnet Lu-Hf geochronology (Ivrea-Verbano Zone, Italy)
Connop, C.H., Smye, A.J., Garber, J.M. and Vervoort, J.D.
Characterizing the formation and evolution of continental lower crust is vital to understanding the Earth’s structure, chemistry, and dynamics. The Ivrea-Verbano Zone (IVZ), northwestern Italy, is considered an archetypal section of continental lower crust, comprising kilometer-scale tracts of metasediments, interspersed with mafic and ultramafic rocks. To better constrain the timescales and timing of emplacement of these lower crustal metasediments as well as their prograde metamorphic history, we present results of a comprehensive garnet Lu-Hf petrochronological investigation.
Garnet-whole-rock Lu-Hf dates from six metapelites, collectively spanning the amphibolite-granulite transition, range from 311 to 239 Ma. Dates show a systematic variation with structural depth in the section, with the shallowest samples recording the oldest Lu-Hf dates and the deepest sample recording the youngest date. Amphibolite-facies samples yield ages of 311.3 ± 2.8 Ma (all error reported as 2σ, MSWD = 3.6), 292.9 ± 2.7 Ma (MSWD = 27), and 275.9 ± 2.5 Ma (MSWD = 9.4) while granulite-facies samples yield dates of 263.9 ± 2.7 Ma (MSWD = 0.7; M91005E02), 275.4 ± 3.1 Ma (MSWD = 3.9), and 238.6 ± 8.2 Ma (MSWD = 0.3).
Garnet in the lowest-grade sample, which records the oldest Lu-Hf date c. 311 Ma, is the only sample that contains remnant prograde growth zoning. Grossular content increases from core to inner-rim (mantle) and decreases toward the outermost rim; almandine and spessartine zoning is the inverse of this general pattern. Garnet isopleths and GASP geobarometry indicate an increase in pressure from 5 to 6.5 kbar and temperature from 500 to 550 °C, from core to mantle. Pseudosection-derived peak metamorphic conditions are 5.5-6.5 kbar and 625-700°C. The outermost garnet rim (less than 50 μm) is defined by well-defined crystal facets, is discordant to inner domains and has distinctive elevated spessartine and depleted grossular concentrations. Isopleths and GASP geobarometry imply equilibration at ~2 kbar, interpreted to be related to the emplacement of the mafic complex [1]. Lu and other HREEs exhibit prograde Raleigh fractionation zoning. Core-enrichment of Lu implies that the Lu-Hf date of 311 Ma records the timing of burial prior to the attainment of peak temperatures.
The remainder of the Lu-Hf dates are significantly younger than published zircon U-Pb constraints on the established age of peak high-temperature metamorphism, ~316 Ma [2], implying that either the Lu-Hf system in garnet records cooling from the thermal climax, or the age of the thermal peak—and attendant garnet growth—is younger than previously thought in the lowermost crust. Most amphibolite- and granulite-grade garnet preserves trace element zoning consistent with growth from a melt phase with superimposed resorption rims. The highest-grade, granulite-facies samples preserve patchy trace element zoning, potentially related to dissolution during post-peak fluid influx.
This dataset shows that: (1) amphibolite-facies sediments were incorporated into the lower crust by some process other than relamination (which would require decompression from higher pressures); (2) the transition from regional compression to transtension, and onset of heating, occurred between 311-~290 Ma, and (3) the lowermost crust remained sufficiently hot to facilitate garnet growth over ~70 Myr; we speculate that these conditions weakened the crust, promoting the onset of Tethyan extension and seafloor spreading (220-160 Ma).