Conference Agenda

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Session Overview
2.4 Magmatic and metamorphic petrology
Monday, 20/Sept/2021:
4:15pm - 5:45pm

Session Chair: Armin Zeh, KIT
Session Chair: Dominik Gudelius, Karlsruhe Institute of Technology

Session Abstract

The Earth’s crust and related mineral resources were formed and modified by magmatic and metamorphic process through Earth’s history. Understanding of these processes in space and time is therefore of fundamental interest for Geoscientist and Society. This session deals with all aspects of magmatic and metamorphic rocks formed through Earth’s history from global to nanoscale. Presentations are welcome dealing with (i) experimental petrology, (ii) field-based studies on magmatic and metamorphic rocks and processes in all kind of geological settings, from crust to mantle and back, (iii) thermodynamic and kinetic modelling, and (iv) related geochronological aspects.

4:15pm - 4:45pm
Session Keynote

Melt inclusions in zircon are powerful petrogenetic indicators and improve zircon thermometry

Dominik Gudelius1, Armin Zeh1, Renat R. Almeev2, Allan H. Wilson3, Lennart A. Fischer4, Axel K. Schmitt5

1Karlsruhe Institute of Technology, Germany; 2Leibniz University Hannover, Germany; 3University of the Witwatersrand, South Africa; 4Albert-Ludwig University Freiburg, Germany; 5Heidelberg University, Germany

Melt inclusions in zircon (MIZ) directly reflect the physicochemical state of the magma during zircon growth. However, their potential as geothermometers and petrogenetic indicators is still poorly explored. Therefore, we investigated MIZ from well-characterized mafic and felsic rocks of the Bushveld Complex (South Africa) and acquired a novel dataset of major, trace and volatile element contents. Re-homogenized MIZ of all rock types display rhyolitic compositions (65-78 wt% SiO2) and similar H2O contents (1.6-4.0 wt%). Liquidus temperatures of MIZ obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment at 930–850°C (at 200 MPa), tailing down to 700°C in some samples. For rutile-bearing mafic cumulates of the lower BC (Marginal and Critical Zone), these temperatures overlap with TiO2 saturation temperatures of MIZ as well as with Ti-in-zircon of host crystals using aTiO2=aSiO2=1.0 [1], in accordance with textural associations of zircon+rutile+quartz. In contrast, MIZ in all rutile-free, magnetite-ilmenite-titanite- and quartz-bearing rocks of the upper BC (Upper Zone ferrogabbros, granitic rocks), display strikingly lower Ti contents, but also higher ƩREE and lower Th/U. Cross-calibration of TiO2 saturation (MIZ) and Ti-in-zircon thermometers with MIZ liquidus temperatures suggests that zircon crystallized at highly reduced aTiO2~0.3, significantly below aTiO2~0.6 previously estimated for rutile-free rocks in the literature, usage of which would underestimate zircon crystallization temperatures by 50-100 °C. In summary, MIZ may inherit chemical signatures of host rocks, are powerful zircon geothermometers and provide constraints for aTiO2 in Rt-free rocks.

[1] Ferry & Watson (2007) CMP 154, 429–437; [2] Hayden & Watson (2007) EPSL 258, 561-568

4:45pm - 5:00pm

Reasons for extreme Th/U zoning of zircon in magmatic rocks: examples from the Bushveld Complex

Armin Zeh1, Dominik Gudelius2, Allan H Wilson3

1KIT, Germany; 2KIT, Germany; 3WiTs, Johannesburg, South Africa

Zircons of magmatic rocks can show enormous variations in Th/U ratios (0.2 to 100) and extreme Th/U zoning. We present data from felsic and mafic rocks of the Bushveld Complex in South Africa. Zircon grains in mafic cumulate rocks reveal Th/U ratios up to 70, those in felsic rocks barely exceed 1.0. In mafic rocks zircon mostly occur together with Rt-Bt-Kfs-Qtz in intercumulus domains, and crystallized during final magma cooling between 900 and 700°C, after >75% of fractional crystallization. The resulting zircons reveal very distinct Th/U zoning trends. Group (1) zircons show systematic increase in Th/U from core to rim (from 0.5 to 20), accompanied by a systematic decrease in U content (from >170 to 10 ppm), group (2) zircons the opposite trend, and group (3) zircons nearly no zoning. Modelling result reveal that all three zoning trends can be explained by minor differences in Bt-Rt-Zrc crystallization history. Trend (1) results from Rayleigh-like fractionation due to zircon growth (±Rt), having different partition coefficients for U ≫ Th. Trend (2) results from zircon growth after onset of biotite-in reaction, causing breakdown of previously formed rutile, thereby releasing U≫Th. Trend (3) results from mass balance constrains, causing mutual compensation of fractionation effects. The absence of pronounced Th/U zoning of zircons in felsic Bushveld rocks also results from compensation of zircon fractionation due to coeval crystallization of abundant rock-forming minerals (Opx-Cpx-Hbl-Pl-Kfs-Qtz) at an early stage of fractional crystallization (10-20%), all being highly incompatible for Th and U [1]. [1] Gudelius et al. (2020). Chemical Geology 546, 119647.

5:15pm - 5:30pm

Differences in decompression of the high-pressure Cycladic Blueschist Unit (Naxos Island, Greece): what can inclusions tell us?

Alexandre Peillod1, Jarosław Majka2,3, Uwe Ring4, Kirsten Drüppel5, Clifford Patten1, Andreas Karlsson6, Adam Włodek3, Elof Tehler4

1Department of Ore Geology, Karlsruhe Institute of Technology, Karlsruhe, Germany; 2Department of Earth Sciences, Uppsala University, Uppsala, Sweden; 3Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Kraków, Poland; 4Department of Geological Sciences, Stockholm University, Stockholm, Sweden; 5Department of Petrology, Karlsruhe Institute of Technology, Karlsruhe, Germany; 6Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden

Determining the tectonic evolution and thermal structure of a tectonic unit that experiences a subduction-related pressure temperature (P-T) loop is challenging. Within a single unit, P-T conditions can vary from top to bottom which can be only revealed by detailed petrological work. We present micropetrological data of the middle section of the Cycladic Blueschist Unit (CBU) in Naxos, Greece, which indicate a different P-T loop than the top of the section.

In the middle section, strong deformation associated with high-T metamorphism erases most of the earlier tectonometamorphic imprints preventing to apply "traditional" geothermobarometry methods. Using Zr-in-rutile and Ti-in-biotite thermometry coupled with quartz-in-garnet elastic barometry and phase equilibrium thermodynamic modeling, we identify a prograde path from ~15.4 kbar to ~19.9 kbar and from ~496 °C to ~572 °C, equilibration during decompression at ~8.3 kbar and ~519 °C followed by near-isobaric heating to ~9.2 kbar and ~550 °C (or even ~584 °C), and a final greenschist-facies equilibration stage at ~3.8 kbar and ~520 °C.

We compare these P-T estimates with published data from the top of the CBU section and find that the bottom half of the CBU on Naxos records higher peak high-pressure (HP) of about 4 kbar than the top, defining the thickness of the CBU to about 15 km in the Eocene. We determine that crustal thickening of up to ~15% occurs in the upper half of the CBU section during exhumation of the HP rocks in an extrusion wedge during convergence.

5:30pm - 5:45pm

Thermobarometry at extreme conditions - what can possibly go wrong? An example

Thorsten Joachim Nagel1, Kathrin Fassmer2

1Aarhus University, Denmark; 2Innsbruck University, Austria

We present eclogites and garnet pyroxenites from Danmarkshavn (Greenlandic Caledonides). So far, one ultra-high pressure (UHP) location has been described from NE Greenland. There, thermobarometry yielded conditions of 972 ºC/36 kbar (Gilotti and Ravna, 2002).

Eclogites from Danmarkshavn show spectacular exsolution of Qtz from Cpx, which is known from UHP assemblages. The sample most suitable for unraveling precise conditions, however, is a garnet pyroxenite containing abundant Cpx and Opx, some Grt, minor accessory minerals as well as little retrograde Am and Pl. Opx and Cpx preserve high-pressure compositions in cores of large crystals and extremely low Al-content in Opx clearly indicate UHP conditions. A considerable portion of these minerals, however, reequilibrated during exhumation with especially Cpx showing complex retrograde zoning. We infer that Grt grains completely reequilibrated during net-transfer reactions producing Am and Pl.

Precise conditions in such rocks are often achieved by intersection of isopleths, e.g. Al-in-Opx with Grt-Cpx-Mg-Fe thermometry. Both isopleth sets have positive slopes in pressure-temperature space. If exhumation occurs along a trajectory steeper than the thermometer isopleth, equilibration of Mg-Fe-exchange during exhumation leads to possibly dramatic overestimation of peak conditions. In our sample, this yields up to 1000 ºC/>40 kbar. Based on Cpx and Opx core compositions alone, however, we infer considerably lower peak conditions of 800-830 ºC/30-32 kbar.

We find that micro-xrf scans of whole thin sections yield powerful data on mineral zoning, reaction progress and the degree of reequlibration. Such maps allow better defining targets for high-resolution mapping and high-precision microprobe work.