此点可见保留辉长岩原始结构的基性岩墙,其中的斜长石具定向生长。Kröner et al.(2006)对其中的锆石进行了离子探针定年。他认为其中保留了部分岩浆锆石和岩浆锆石+变质增生边以及变质锆石。据此,Kröner et al.(2006)认为两个~1915 Ma (图III-8)的年龄值解释为原岩(侵入岩墙)形成的年龄。新的定年数据表明其变质年龄介于1.85-1.96Ga。
The stop is situated at the foot of the mountain [lat. 39º 06' 43.4"; long. 113º 14' 2.0"] with a temple called Guifengsi (Fig. 35). Exposed here is the Jingangku ‘formation’ of the ‘lower’ Wutai sequence. One of the most characteristic features of metamorphic mineral assemblages in this outcrop is the presence of kyanite +gedrite or kyanite + chlorite in the rocks within a contact zone between the meta-ultramafic to mafic rocks and metapelites. Meta-ultramafic to mafic rocks are exposed in the ridge of the mountain, including serpentinites, talc schist, tremolite schist, actinolite schist, anthophyllite schist, amphibolite and various greenschists. Between the meta-ultramafic to mafic rocks and metapelites is a transitional zone that is located on the mountain slope and comprises kyanite-gedrite schist, kyanite-phengite-chlorite greenschist, kyanite-gedrite-staurolite-garnet schist, kyanite-tourmaline-phengite greenschist, and kyanite-gedrite-cordierite schist. Based on the available petrogenetic grids, Wang et al. (1996) estimated that these assemblages formed at pressures of 0.9-1.4 Gpa. However, another alternative interpretation is that the formation of kyanite-gedrite assemblages may be related to the interaction (e.g. fliud metasomatism) of the meta-ultramafic to mafic rocks with the metapelites. Because of time limitations, we cannot go up to the mountain to examine all these rock assemblages. What we will examine at this stop are garnet porphyroblasts -bearingmica schist, and kyanite-gedrite schist where gedrite crystals do not show any preferred orientation, indicating post-tectonic growth.
This is a quarry opened-up in the Ekou granite (Fig. 35), which outcrops over an area of 30 Km2. The granite is in tectonic contact with rocks of the Wutai Complex, exposed at the rear of the nearby iron ore processing plant. The rock is chiefly a pink, medium grained, deformed granitoid with a strong gneissic fabric. Elongate ribbons of strongly undulose quartz alternate with layers composed of perthitic microcline, plagioclase, biotite and rare garnet.
Liu et al. (1985) initially obtained a some-what imprecise U-Pb multigrain zircon age of 2520 ± 30 Ma for the Ekou Granite. Based on the simple interpretation that 2520 Ma was the real age and the belief that the granite intruded the Wutai Complex, it has generally been considered that the Ekou Granite is younger than the Wutai Complex. This interpretation, and the highly siliceous nature of the rocks, led Wang et al. (1992) to conclude that the Ekou Granite is related to late, post-collisional events in the area. Several samples of Ekou Granite were collected for analysis, of which only two provided suitably concordant data points to enable the age to be determined (Wilde et al., 1997). In both samples, the number of concordant zircon grains is small, reflecting the extensive disturbance to the isotopic systems and explaining the large uncertainties in the multigrain zircon data. The zircon crystals are mostly dark brown in colour with numerous inclusions and brown discolouration patches. Sample PC-95-33 (lat. 39° 06' 30"; long. 113° 14° 48") defines a 207Pb/206Pb age of 2566 ± 13 Ma, based on 5 concordant analyses (Fig. 36). The data for sample 95-19 (Fig. 37) from the same location are more tightly constrained and 10 analytical points define a 207Pb/206Pb age of 2555 ± 6 Ma. These latter are considered to be a better representation of the true age of the granite, which was emplaced ~2555 Ma ago.
The results indicate that the Ekou Granite is older than the felsic volcanic rocks of the Middle Wutai Complex, with ages of ~2525 Ma (see later section) and we now interpret this granite as forming prior to development of the Wutai arc, possibly at the western margin of the Eastern block of the North China Craton (Zhao et el., 2001a).
Stop 11. 岩头乡政府路边,复杂褶皱群
Stop 12. 太平沟公路旁;[内容]:枕状熔岩
On a roadside [lat. 39º01′08.1"; long. 113º19′44.2"]~500 meters south of Taping Village (Fig. 35), we will examine the pillow lava metabasalts from the Baizhiyan Formation of the Middle Wutai sequence. Delineating the pillows are 1~2 cm epidote-rich rims surrounding fine-grained 0.1-0.3 m-wide pillow cores, which consist of chlorite, albite carbonates, epidote, and opaque minerals. Most pillows have been elongated and flattened to form a foliation parallel to the regional schistocity (079/60°). Some pillows show a flat lower edge and a convex upper edge, possibly indicating way-up structures.