Metamorphism protolith

The isotope composition of metamorphic rocks is mainly controlled by three factors, besides the temperature of exchange (1) the composition of the pre-metamorphic protolith, (2) the effects of volatilization with increasing temperatures and (3) an... 

The principal control on the chemical composition of a metamorphic rock is the composition of the pre-metamorphic protolith. Sometimes metamorphic recrystallization ma y be isochemical but most commonly there is a change in chemical composition (Figure 1.3). This is principally controlled by the movement of fluids and the diermd metamorphism. Metamorphism is frequently... 

The Kokchetav Massif of northern Kazakhstan is a very large, fault-bounded metamorphic complex of Late Proterozoic-Paleozoic protolith age, surrounded by the Caledonian rocks of the Ural-Mongolian fold belt. The Kokchetav UHP and HP belt runs NW-SE extending at least 150 km long and 17 km wide. This massif has attracted much interest since the discovery of metamorphic diamonds. It is the first locality where microdiamonds were found within metamorphic rocks derived from crustal material. 

Hoskin P. W. O. and Black L. P. (2000) Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon. J. Metamorph. Geol. 18(4), 423 -439. 

Numerous petrological studies have established that the protoliths of eclogites underwent progressive dehydration during prograde metamorphism, and that many were later rehydrated to... 

Additional evidence of limited fluid mobility during prograde HP and UHP metamorphism is seen in the failure of dissimilar protoliths to achieve stable isotope equilibrium on mm-, cm-, and m-scales despite extreme metamorphic conditions (Sharp et al., 1993 Rumble and Yui, 1998 Baker et al., 1997 Zheng et al., 1998a,b, 1999, 2003 Fu et al., 1999 Philippot and... 

Fan Q., Liu R., Ma B., Zhao D., and Zhang Q. (1996) Protolith and ultrahigh-pressure metamorphism of Maowu mahc-ultramahc rocks, Dabieshan. Acta Petrol. Sin. 12, 29 -47. 

Figure 21 MORE normalized spidergrams for mafic garnet granuUtes from the Tonsina area, at the base of the Talkeetna arc section at 1 GPa, 1,000 °C (DeBari and Coleman, 1989 Kelemen et al., 2003a). Bold red line is average from Table 3. These orthogneisses formed via metamorphic recrystaUization of protoliths with major and trace-element contents identical to gabbronorites from higher in the Talkeetna section. Garnet in these rocks is metamorphic, as previously proposed (DeBari and Coleman, 1989). Data from our unpublished research.

Dehydration is the best-known and most common example of metamorphic volatilization. The magnitude of the isotopic effect is controlled by the amounts of water expelled high-grade rocks contain less water than lower grade equivalents due to progressive dehydration (Spear 1993). In contrast to shales, which are the most water-rich protolith commonly available for metamorphic dehydration (up to 5 wt %), igneous rocks typically contain only 0.5 to 0.8 wt % H2O (Wedepohl 1969). 

The whole-rock Rb-Sr date of 1789 141 Ma (Paleoproterozoic) is the time when these rocks were recrystallized during regional metamorphism to amphibolite grade and the strontium they contained was isotopically homogenized so that all three specimens had the same initial Sr/ Sr ratio of 0.7059 0.0041. This value indicates that their protoliths contained strontium derived from the lithospheric mantle in the form of volcanic rocks or their weathering products. 

---