Ultrahigh‐pressure metamorphism and decompressional P–T paths of eclogites and country rocks from Weihai, eastern China: Reply

Abstract

We welcome the constructive comment by Enami on our suggested in situ model for coeval UHP metamorphism of coesite-bearing eclogite, garnet amphibolite, and country rock gneisses at Weihai in the northeastern tip of the Su-Lu UHP terrane (20). Establishment of UHP metamorphism for this region was based on the occurrence of coesite relics (17; 14; 20) from a single eclogitic block at a beach locality. We have recently identified two additional coesite-bearing eclogite localities in the vicinity of Weihai (15). We agree with Enami that the evidence for UHP metamorphism preserved in the country rock gneisses is `far less convincing'. In this area, we described several small garnet amphibolite layers or pods and calc–silicate layers concordant with the surrounding granitic gneisses. The amphibolites preserve mineral aggregates after omphacite and show ghost eclogitic textures (Fig. 2f, Zhang et al. 1995a). Moreover, plagioclase in some garnet-bearing clinopyroxene gneisses is clearly a retrograde phase (Fig. 3a, 20); a P–T estimate of the peak metamorphism was not made, but these rocks were assumed to have experienced similar P–T histories to the less thoroughly overprinted coesite-bearing eclogite block. We used the term `calc–silicate rock' but not `calc–silicate gneiss'; the calc–silicate rocks are not `similar to typical skarns' which tend to occur at the contact between carbonate wall rocks and granitic intrusives. The assemblage of the calc–silicate rocks described in our paper (20) consists of Grt + Cpx ± Qtz + Ttn (Fig. 1) rather than Pl + Scp + Grt + Cpx + Ep. As shown in Fig. 1, garnet, clinopyroxene, and quartz constitute up to 95–97 vol% of the rock; minor secondary anhedral plagioclase and zoisite rim garnet and clinopyroxene (Fig. 2). Garnet and clinopyroxene contain titanite and apatite inclusions. The plagioclase contains inclusions of relict garnet and titanite, and ranges in An content from 19 to 65; these anhedral plagioclase grains are not in equilibrium with the precursor garnet and clinopyroxene, and appear to be a retrograde phase. Photomicrograph showing retrograde zoisite after clinopyroxene in calc-silicate rock (SL91-6E). Crossed polars, width of view = 0.75 mm. Cpx, clinopyroxene; Zo, zoisite; Grt, garnet. (b) A Schematic diagram granoblastic texture and a typical assemblage Grt + Cpx + Ttn + Qtz for (a) a calc-silicate rock (SL91-6B) from Weihai (width of view = 1.5 mm). Grt, garnet; Cpx, clinopyroxene; Ttn, titanite; Qtz, quartz. Lack of plagioclase sets the minimum P for the peak metamorphism at 14 kbar. As described in detail in our original paper (20), the calculated maximum P could be as high as 30–35 kbar. Such a P–T estimate is consistent with the occurrence of high-Al titanite which contains 5–6 wt% Al2O3 (Table 3, 20), and is similar to high-Al titanite in the Donghai coesite-bearing eclogite of the Su-Lu region (21), in the Shuanghe carbonate-bearing eclogite in the Dabie Mountains (1), and in UHP eclogite of the Western Gneiss Region (11). These observations indicate the eclogitic and country rocks at Weihai share the same P–T path (20). Both we and Enami recognize an upper amphibolite-to-granulite facies overprint for the Weihai occurrence. We agree with him that the lack of relict coesite in the country rock gneisses means that the assignment of UHP conditions remains questionable. Our interpretation, however, is plausible, based on concordant layering, high-Al titanite, and P–T computations. For kinetic reasons, with the influx of minor amounts of fluid into the gneissic rocks, any earlier formed coesite grains would have been entirely transformed to quartz at such high-T conditions unless the coesite relics were stored in rigid crystals such as zircon. It took more than half a century for European and Russian geologists to discover tiny inclusions of diamond and coesite in strong, chemically resistant container grains of garnet, clinopyroxene, and zircon from UHP metamorphic rocks in collision belts within Eurasia and Africa (567a; 2). With new appreciation and sophisticated instrumentation, the UHP record in felsic gneisses of the Dabie–Su-Lu region have been increasingly recognized. This includes the additional finding of tiny coesite inclusions in zircons from Dabie felsic gneisses (12; 13), in dolomite and garnet from calc–silicate rocks and dolomite-bearing eclogite (10; 19), and in garnet and jadeite from jadeite-bearing quartzite (21; 8O values have been determined for UHP minerals from country rock gneiss, quartzite, and eclogite in the Qinglongshan of the Su-Lu terrane (9). Aegirine as a retrograde phase after jadeite and garnet in garnet–jadeite–quartz rocks have been reported (18; 21, 8); it has been increasingly recognized in country gneiss of the Su-Lu terrane (16; 21; 4) and in the Dabie Mountains (3). The identification of additional UHP indicators will require further studies; the debate regarding the in situ versus foreign origin for eclogitic rocks will continue. We look forward to further discoveries of phenomena constraining the P–T conditions attending UHP metamorphism of the Weihai rocks.