Tectonothermal
evolution of Archaean basement rocks from the Eastern zone of the North
China craton:
implications
for mantle plume tectonics
Guochun Zhao, S.A. Wilde & P.A. Cawood
Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, Perth, WA 6001, Australia
The eastern zone of the North China Craton consists mainly of pre-tectonic high- and low-grade gneisses, syn-tectonic granitoids and minor amounts of ultramafic (komatiitic) to mafic volcanic and sedimentary supracrustal rocks. High-grade gneisses dominate the Eastern Hebei, Miyun-Chengde, Eastern Shandong, Northern Liaoning and Southern Jilin domains, and low-grade gneisses (granite-greenstone belts) occupy the Western Shandong, Southern Liaoning and Anshan—Benxi domains. In both high- and low-grade domains, gneisses and granitoids make up more than 80% of the total exposure. Supracrustal rocks, although volumetrically subordinate, are present throughout the domains.
Basement rocks from all domains in the eastern zone of the North China Craton, regardless of their ages, metamorphic grades and compositions, show substantially similar metamorphic characteristics. They are all characterized by anticlockwise P—T paths. The similarity in evolution of metamorphic rocks from these domains is clearly not an artefact of thermobarometry, since the inferred P—T paths are well constrained by metamorphic reaction textures, as well as by the P—T estimates. Most mafic granulites, amphibolites and some pelitic rocks in the eastern zone preserve the prograde, peak and post-peak nearly isobaric cooling textures. The prograde metamorphic textures are indicated by inclusions within minerals developed at the peak stage, and are represented by assemblages of hornblende + plagioclase + quartz ± biotite in mafic granulites, chlorite + actinolite + epidote + plagioclase + quartz in amphibolites and biotite + plagioclase + quartz in pelitic gneisses. The peak stage is shown by assemblages of orthopyroxene + clinopyroxene + garnet + plagioclase + quartz from the mafic granulites, hornblende + plagioclase + quartz + garnet from garnetiferous amphibolites and garnet + sillimanite + plagioclase + quartz + biotite from pelitic gneisses. Near-isobaric cooling textures are exhibited by garnet + quartz symplectic coronas in mafic granulites, actinolite + garnet (rim) retrogressive rims around garnet or hornblende grains in amphibolites, and kyanite replacing sillimanite or staurolite replacing sillimanite + garnet in pelitic gneisses. These textural relations and their P—T estimates define anticlockwise P—T paths for the basement rocks in the eastern zone.
The calculated anticlockwise P—T—t paths reflect an origin related to the intrusion and underplating of large amounts of mantle-derived magmas that not only provide heat for the metamorphism but also add a large volume of mostly mafic material to the base of the crust. Large volumes of underplating magma leading to metamorphism with an anticlockwise P—T path may occur in continental magmatic arc regions (Wells, 1980; Bohlen, 1991), mantle plume tectonic regimes (hot-spots) (Bohlen, 1991) and continental rift environments (Sandiford and Powell, 1986). The continental rifting and continental magmatic arc models are inappropriate for the origin of the basement rocks in the eastern zone as they cannot explain the widths of exposed basement rocks, dominant oval structures and lack of intrusion of abundant mafic dykes which are commonly associated with rifting and continental magmatic arc regions. And also, the continental rifting and continental arc models cannot reasonably explain the occurrence of anomalously high-temperature komatiitic rocks in the basement of the North China Craton. Alternatively, we favor a mantle plume (hot-spot) model to explain the origin of the basement rocks of the eastern zone of the North China Craton. Combining the mantle plume thermal structure with the available lithological, structural, metamorphic and geochronological data, we propose the following tectonic scenario for the formation of the basement rocks in the eastern zone of the North China Craton:
(1) In the Early Archaean there was a primitive sialic continental crust which is represented by the granitic gneiss of c. 3800 Ma in the Anshan domain and fuchsite quartzite of c. 3800 Ma in the eastern Hebei domain. The source and initial location of their material is unknown.
(2) In the Mid-Archaean, enormous volumes of ultramafic to mafic volcanic rocks and pre-tectonic TTG plutons were produced owing to the interaction of upwelling mantle plumes and lithosphere. The huge plume head uplifted the mantle lithosphere and overlying continental crust and caused lithospheric stretching, leading to extensive eruption of ultramafic to mafic volcanism (komatiites and basalts). At the same time, the heat transfer from the plumes to the upper mantle or lower crust resulted in extensive partial melting of basaltic or amphibolitic rocks to form large volumes of light TTG magma which becomes unstable at the base of the crust and rises diapirically into the lower and upper crust, forming the domes. The relief associated with the diapiric intrusion of the TTG plutons led to local erosion and sedimentation, which is represented by minor amounts of sedimentary supracrustal rocks in the zone.
(3) In the Late Archaean, another episode of mantle plumes moved to the base of the crust, causing local volcanism, but did not result in large volumes of additional TTG magma, since the low-melting component had been extracted from this section of the crust during the last plume event (see Hill et al., 1992). Significant geological consequences of this plume event are extensive regional metamorphism and the production of syn-tectonic granitoids. At first, the relatively cooler plume head heated the crust, causing prograde metamorphism (M1). The crust close to the plume head underwent amphibolite-facies metamorphism, as in the high-grade domains, while the crust further away from the head underwent greenschist-facies metamorphism, as in the low-grade domains. At the same time, large volumes of magma were added to the base of the crust and may intruded to higher levels, resulting in crustal thickening. Subsequently, the hot plume "tail" heated the crust, causing peak metamorphism (M2) at amphibolite to granulite facies, depending on the distance to the plume, and widespread anatexis of TTG gneisses, which resulted in the production of syn-tectonic granitoids. Finally, the heated crust experienced near-isobaric cooling (M3) when the effect of heating ceased through the termination of plume activity. This tectonothermal process is consistent with the anticlockwise P—T paths estimated from the basement rocks in the eastern zone of the North China Craton.
References
Bohlen, S.R., 1991, On the formation of granulites: Journal of Metamorphic Geology, v. 9, p. 223—229.
Hill, R.I., Campbell, I.R., Davis, G.F., and Griffiths, R.W., 1992, Mantle plumes and continental tectonics: Science, v. 256, p. 186—193.
Sandiford, M., and Powell, R., 1986, Deep crustal metamorphism during continental extension: ancient and modern examples: Earth Planetary Science Letter, v. 79, p. 151—158.
Wells, P.R.A., 1980, Thermal models for magmatic accretion and subsequent metamorphism of continental crust: Earth Planetary Science Letter, v. 46, p. 253—265.