Metamorphic Evolution of Garnet Amphibolite from the Yaganbuyang Area in the South Altyn Orogen,West China:Insights from Phase Equilibria Modeling and Geochronology

Garnet amphibolite is one of the common metabasic rocks exposed in collisional orogenic belt,the metamorphic evolution of which is associated closely with orogenic processes.The Yaganbuyang garnet amphibolites occur as blocks hosted by massive granitic gneiss,and consist mainly of hornblende,garnet,clinopyroxene,plagioclase,biotite,quartz with minor rutile/ilmenite and phengitic muscovite.These garnet amphibolites were interpreted to have experienced decompression-dominated evolution that can be divided into three generations(M1,M2,M3),based on the petrographic observations and phase equilibria modeling calculated by THERMOCALC.The assemblage of the first generation(M1)is inferred to possibly be dominated by garnet+omphacite+rutile+phengite+quartz,which is modeled to be roughly stable at P>25 kbar and T>800℃.The second generation(M2)is characterized by the local symplectites of clinopyroxene+plagioclase produced from omphacite,indicating a near-isothermal decompression from~23.8 kbar/875℃to~10 kbar/852℃.The third generation(M3)is marked by the kelyphitic rims of plagioclase+hornblende around garnet and of hornblende+ilmenite around clinopyroxene,involving the late-stage retrogression from~9.8 kbar/848℃to~5.8 kbar/645℃.Zircon U-Pb dating yielded one group metamorphic age of c.500 Ma that is interpreted to represent the timing of the peak eclogite-facies metamorphism.A combination of petrography observation,phase modeling results and geochronology data suggests that the Yaganbuyang garnet amphibolites have once undergone eclogite-facies metamorphism by continental subduction rather than crustal thickening.Therefore,the Yaganbuyang area is an eastward extension part of the South Altyn HP-UHP metamorphic belt.

Phase Equilibria Modeling and P-T Evolution of the Mafic Lower-Crustal Xenoliths from the Southeastern Margin of the North China Craton

The Precambrian lower crust rocks at the southeastern margin of the North China Craton （NCC） are mainly exposed as granulite xenoliths hosted by Mesozoic dioritic porphyry and metamorphic terrains in the Xuzhou-Suzhou area. Garnet amphiholites and garnet granulites are two kinds of typical lower-crustal xenoliths and were selected to reconstruct different stages of the metamorphic process. In this study, in view of multistage metamorphic evolution and reworking, phase equilibria modeling was used for the first time to better constrain peak P-T conditions of the xenoliths. Some porphyroblastic garnets have a weak zonal structure in composition with homogeneous cores and were surrounded by thin rims with an increase in XMg and a decrease in X Ca （or X Mg）- Clinopyroxene contain varying amounts of Na2O and Al2O3 as well as amphibole of TiO2, while plagioclases are different in calcium contents. Peak metamorphic P-T conditions are calculated by the smallest garnet x（g） （Fe2＋/（Fe2＋＋Mg）） contours and the smallest plagioclase ca（pl） （Ca/（Ca＋Na）） contours in NCFMASHTO （Na2O-CaO-FeO-MgO-Al2O3-SiO2- H20-TiO2-Fe2O3） system, which are consistent with those estimated by conventional geothermobarometry. The new results show that the peak and decompressional P-T conditions for the rocks are 850-900 ℃/ 1.4-1.6 GPa and 820-850 ℃/0.9-1.3 GPa, respectively, suggestive of high and middle-low pressure granulite-facies metamorphism. Combined with previous zircon U-Pb dating and conventional geothermobarometry data, it is indicated that the xenoliths experienced a clockwise P-T-t evolution with nearisothermal deeompressional process, suggestive of the Paleoproterozoic subduction-collision setting. In this regard, the studied region together with Jiao-Liao-Ji belt is further documented to make up a Paleopro- terozoic collisional orogen in the eastern block of the NCC.

Prograde and retrograde growth of monazite in migmatites:An example from the Nagercoil Block,southern India

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900 ℃. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 ℃. SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae- oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

Re-evaluating monazite as a record of metamorphic reactions

This study presents a re-examination of historical specimens(DG136 and DG167)from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element(REE)distribution between garnet and monazite(and other accessory minerals)during metamorphism.Nine-hundred and fifty-one new monazite petrochronology spot analyses on 29 different grains across two specimens outline detailed(re)crystallization histories.Trace element data collected from the same ablated volume,interpreted in the context of new phase equilibria modelling that includes monazite,xenotime and apatite,link ages to specific portions of the pressure-temperature(P-T)paths followed by the specimens.These linkages are further informed by garnet Lu-Hf geochronology and xenotime petrochronology.The clockwise P-T paths indicate prograde metamorphism was ongoing by ca.80 Ma in both specimens.The structurally deeper specimen,DG136,records peak P-T conditions of~755-770℃and 8.8-10.4 kbar,interpreted to coincide with(re-)crystallization of low Y monazite at~75-70 Ma.Near-rim garnet isopleths from DG167 cross in the observed peak assemblage field at~680℃ and 9.3 kbar.These conditions are interpreted to correspond with low Y monazite(re-)crystallisation at~65 Ma.Both specimens record decompression along their retrograde path coincident with high Y 70-55 Ma and 65-55 Ma monazite populations in DG136 and DG167,respectively.These findings broadly agree with those initially reported~20 years ago and confirm early interpretations using trace elements in monazite as generally reliable markers of metamorphic reactions.Modern phase equilibria modelling and in situ petrochronological analysis,however,provide additional insight into monazite behaviour during anatexis and the effects of potential trace element buffering by REE-bearing phases such as apatite.

Deformation induced decoupling between U-Pb and trace elements in titanite revealed through petrochronology and study of localized deformation

In this contribution,we analyzed a pair of mafic samples collected from a recently identified shear zone and its proximal footwall from the Manicouagan Imbricate Zone(MIZ)of the central Grenville Province,Québec,Canada.Titanite petrochronology,metamorphic phase equilibria modelling,trace element thermometry,and electron backscattered diffraction data were used to define a Pressure-Temperature-timeDeformation path for the two samples.An interconnected dislocation network within titanite grains,as outlined with Kerneled Average Misorientation maps,are spatially correlated with variation in the U-Pb system but not with that observed for trace element These results suggest that the U-Pb system was decoupled from trace and rare earth elements and that deformation,rather than interface-coupled dissolution-precipitation reactions or re-crystallisation,was the main driver for this decoupling.In addition to highlighting a potential pitfall of titanite petrochronology,our P-T-t-D path reveals that ductile shear zones were active later than previously suggested within the MIZ.