Construcural geometry and kinematics of the Dabie Mountains Orogen

Author: Xu Shutong et al. / Country:
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Publish Date: 2002-06-01
Features: 1.5.1.1.6 Mineral Assemblages of Generations and Metamorphic Evolution According to the relationships between different minerals and their formation conditions, six generations of mineral assemblages can be distinguished: (1) Quartz, rutile, epidote, sodium mica, and glaucophane, representing the greenschist-blueschist facies metamorphism before garnet-peridotite, which Castelli et al. (1998) did not classify as a metamorphic stage. (2) Quartz, omphacite, garnet, kyanite, and rutile, representing the early garnet-peridotite facies stage, equivalent to stages I, II, and III of Castelli et al. (1998), with estimated P-T conditions of 2.0–2.4 GPa and 700–720°C. (3) Garnet, omphacite, kyanite, rutile, and scapolite, representing the peak garnet-peridotite metamorphism, with inferred P-T conditions of ~4.0 GPa and 750°C (Liu Xiaochun et al., 1999). Castelli et al. (1998) did not classify this stage due to differing views on the occurrence conditions of scapolite. (4) Clinozoisite/epidote, sodium mica, and glaucophane, Ca-Na amphibole, and plagioclase, representing the early decompression retrograde stage, equivalent to stage IV of Castelli et al. (1998), with estimated P-T conditions of 1.4 GPa and 700°C. (5) Ca-Na amphibole (augite-plagioclase post-metamorphic symplectite) (corundum-plagioclase post-metamorphic symplectite) (spinel-plagioclase post-metamorphic symplectite), representing the hornblende facies isothermal retrograde stage, equivalent to stage V of Castelli et al. (1998), with estimated P-T conditions of 0.8 GPa and 700°C. (6) (Hornblende-albite post-metamorphic symplectite) albite, chlorite, and staurolite, representing the greenschist facies retrograde stage, equivalent to stage VI of Castelli et al. (1998), with estimated P-T conditions of T=0.5 GPa and P. Based on this, it is inferred that the Zhujiachong garnet-peridotite has undergone ultra-high-pressure metamorphism (Figure 1-29).
1.5.1.2 Matrix The matrix of Zhujiachong garnet-peridotite is two-mica gneiss, intercalated with varying-scale mylonite bands. The main mineral components include plagioclase, quartz, biotite, muscovite, alaskite, epidote, greenstone, garnet, kyanite, and a small amount of rutile, which may have experienced garnet-peridotite facies metamorphism. Among them: (1) Garnet, rutile, and alaskite may be residual garnet-peridotite facies minerals. (2) Plagioclase, biotite, muscovite, and epidote form an amphibolite facies retrograde assemblage. (3) Chlorite, albite, and staurolite form a greenschist facies retrograde assemblage. Recently, Liu Jingbo et al. (2001) discovered coesite inclusions in zircons from alaskite gneiss near Zhujiachong, which indirectly proves that the garnet-peridotites and some gneisses in this belt have undergone ultra-high-pressure metamorphism.
1.5.2 Western Section: Xuanhuadian and Hong'an Garnet-Peridotite Since no ultra-high-pressure indicators have been found in these garnet-peridotites (Zhang Zeming et al., 1991), You Zhen Dong et al. (1998) high-pressure garnet-peridotite in these areas, including Xiongadian. The garnet-peridotites occur in schist, gneiss, and leucogranite.
1.5.2.1 Petrography The main mineral components include garnet, omphacite, and rutile of garnet-peridotite facies assemblage, as well as pre-garnet-peridotite facies minerals such as jadeite, epidote, alaskite, quartz, and sodium feldspar, and retrograde minerals such as glaucophane, calcic amphibole, sodium feldspar, and frozen glaucophane.
1.5.2.1.1 Garnet In most cases, the garnet content in these garnet-peridotites exceeds 40%, semi-autogenic to autogenic, with diameters of 0.5–2 mm, containing a large number of pre-garnet-peridotite facies inclusions such as jadeite, epidote, alaskite, quartz, and sodium feldspar. These inclusions are sometimes oriented, forming residual pre-garnet-peridotite foliation. The garnet rim contains quartz, rutile, and omphacite inclusions, while the garnet core is rich in MnO, and the rim is rich in MgO and FeO, representing prograde metamorphic component zoning. Garnet is mainly iron-aluminum garnet. In blue-kyanite garnet-peridotites, garnet contains more magnesium-aluminum garnet, which may lack inclusions or only have quartz inclusions, without compositional zoning (You Zhen Dong et al., 1998).
1.5.2.1.2 Omphacite The content of omphacite is usually around 40% (including retrograde post-metamorphic symplectite). Some omphacite is inclusions in garnet of garnet-peridotite, while some omphacite contains rutile, garnet, or glaucophane inclusions. Most of them have retrograded to low jadeite jade (Jd11–27% Aug, 9%–67% ACm, 9.22%), forming unstable assemblages with amphibolite facies minerals (Eide, 1995). Sometimes they are short or long prismatic, without mineral inclusions, with high SiO? content (55.48%–55.74%) and Jd content (47%–48%) (Zhang Zeming et al., 1991).
1.5.2.2 Metamorphic Processes The metamorphic processes include four stages: sodium feldspar-epidote-amphibolite (pre-garnet-peridotite) facies, garnet-peridotite facies, epidote-blueschist facies, and greenschist facies (You Zhen Dong et al., 1998). Eide (1995) identified four metamorphic stages in Hong'an garnet-peridotite: Stage I is represented by sodium amphibole, sodium-calcium amphibole, or calcium amphibole inclusions in garnet, along with epidote, alaskite, quartz, and rutile, indicating the prograde metamorphic assemblage before garnet-peridotite facies. Stage II is the garnet-peridotite facies mineral assemblage: garnet, omphacite, rutile, sodium or sodium-calcium amphibole, quartz, and minor alaskite, muscovite, and kyanite. Stage III has a mineral assemblage of staurolite, titanite (replacing rutile), epidote, calcium amphibole (replacing garnet), calcium-rich amphibole (replacing sodium or sodium-calcium amphibole), sodium amphibole, sodium feldspar, quartz, or pyroxene (low jadeite component), and sodium amphibole (post-metamorphic symplectite of omphacite), representing retrograde amphibolite facies. Stage IV has a mineral assemblage of epidote, sodium feldspar, quartz, and staurolite, or biotite and chlorite (replacing garnet), representing high greenschist facies retrograde metamorphic conditions. Although the calculated peak metamorphic conditions are P=2.0 GPa, recent reports (Li Shuguang, 2001) indicate that the zircon U-Pb ages of Xuanhuadian garnet-peridotite are approximately 700 Ma and 230 Ma. The former may represent the protolith age, while the latter is the metamorphic age.
1.5.2.2 Matrix The matrix mainly consists of gneiss, schist, and leucogranite, intercalated with varying-scale mylonite bands. Gneiss is widely distributed, primarily as white feldspar-albite gneiss, with main mineral components of sodium feldspar, potassium feldspar, quartz, and muscovite, and minor minerals such as garnet, epidote, chlorite, and biotite, intercalated with mylonite bands rich in mica. Schist is less common, primarily occurring as interlayers of gneiss, most of which are strongly sheared mylonite bands. Based on mineral composition, they can be divided into garnet-epidote-muscovite-quartz schist, epidote-muscovite or biotite-quartz schist. The former may have undergone garnet-peridotite facies metamorphism (Eide, 1995).
Leucogranite The characteristics of leucogranite are high SiO? content (77%) and K?O+Na?O content of 9.12%. The protolith may be acidic intrusive rock (Zhang Zeming et al., 1991).
1.5.2.3 Tectonic Setting of the Garnet-Peridotite Belt As a unified garnet-peridotite belt (including the Qianshan-Yingshan-Xinxian garnet-peridotite belt and the Taihu-Hong'an-Xuanhuadian garnet-peridotite belt), apart from differences in the proportion of different rock components, the absence of granulite facies overprinting during metamorphism, and the properties of ultra-high-pressure minerals, other aspects are very similar to ultra-mafic rock belts. The heterogeneity of different tectonic settings and lithological rocks is more pronounced. For example, in terms of rock types, in addition to garnet-peridotite, there are ultra-mafic rocks such as garnet-peridotite and garnet-olivine peridotite in the Shima, Maowu belt (Okay, 1993), serpentine near Baiyangling in Shuanghe, in Mao near Huangpu, and garnet-olivine peridotite in the Bixiling belt (Zhang Cifal, 1995), etc. Metamorphic rocks include marble, quartz-jadeite rock, and slate in the Changpu area. Most of these rock blocks belong to garnet-peridotite facies (including marble and quartz-jadeite rock), some to amphibolite facies such as metamorphic granite, and a few to greenschist facies such as slate. Their tectonic settings are respectively mantle-derived, crust-derived, and crustal rocks. Most matrix gneiss is retrograded garnet-peridotite facies, such as two-mica gneiss, and a small part is amphibolite facies, such as alkaline sodium feldspar granite gneiss.