Plate tectonic evolution and mineral resource potential of the Lancang River zone, Southwestern Yunnan, People's Republic of China [Elektronische Ressource] / vorgelegt von Klaus Heppe

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• Publié par: georg-august-universitat_gottingen
• Publié le: 01 janvier 2005
• Nombre de lectures: 18
• Langue: English
• Poids de l'ouvrage: 10 Mo

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Plate Tectonic Evolution
and Mineral Resource Potential
of the Lancang River Zone, Southwestern Yunnan,
People’s Republic of China
Dissertation
zur Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen Fakultäten
der Georg-August-Universität zu Göttingen
vorgelegt von
Klaus Heppe
aus Warstein (NRW)
Göttingen 2004D 7
Referent: Prof. Dr. B. Lehmann
Korreferent: Prof. Dr. D. Bannert
Tag der mündlichen Prüfung: 9. Juni 2004Abstract
The geology of West Yunnan plays a key role in understanding the tectonic develop-
ment of Southwest China and bordering countries. In the scientific literature, not only the
Himalayan (cf. Tapponier et al., 1982, 1986; Dewey et al., 1988, 1989; Houseman and
England, 1993; Leloup et al., 1995; Wang and Burchfiel, 1997), but also the Late
Paleozoic and Mesozoic geodynamic evolution of South and West Yunnan is the subject
of heated debate. In every case, there is general agreement that the present geological
situation is primarily attributable to the evolution of the Tethys (Hsü and Bernoulli, 1978,
Sengör, 1979, 1981, 1982) and the assumption that Southeast Asia constitutes a mosa-
ic of Gondwana-derived terranes (e.g. Bunopas and Vella, 1978; Mitchell, 1981; Sengör,
1985; Hutchison, 1989; Metcalfe, 1988, 1996, 1998). Although investigations constrain-
ing the timing of rift, drift and collision have been of prime importance within the last two
decades (e.g. Helmcke and Lindenberg, 1983; Helmcke, 1984, 1985; Huang et al., 1984;
Metcalfe, 1993, 1995; Zhong et al., 1999; Zhong et al., 2000; Chonglakmani et al., 2001),
the crucial question of how many micro-continents split off from Gondwana, drifted in the
Tethys and collided with Eurasia, has not been satisfactorily answered. In this debate,
the Lancang River Zone (Lancangjiang) is believed to hold a key position in tracing the
major divide between Gondwana and Eurasia and has been the subject of a range of
interpretations since the early 1980’s. This study re-evaluates the existing data and pres-
ents an integrated tectono-sedimentological, geochronological and geochemical
approach to the Lancang River Zone. The Late Paleozoic and Mesozoic geodynamic
evolution and its mineral resource potential can be summarized as follows:
[1] A near-shore continental rift or back-arc basin with strong bathymetric variation must
have existed from the Upper Devonian to the middle Lower Permian along the present
Lancang River Zone. The extension phase, which led to the creation of the basin, is in
time and facies related to the expanding continental margin of the Yangtze Platform in
the Devonian and Lower Carboniferous (Feng and Helmcke, 2001, Heppe, 2002).
During the rifting phase, basic to acid volcanic rocks were extruded in a submarine
environment to form small-scale Kuroko-type volcanogenic massive sulfide (VMS)
deposits. They form a belt along the central and southern Lancang River. Deposits of
this type are Sandashan, Yinzishan, Dapingzhang and Xincun.
[2] Characterized by an eastward progressing deformation front, closure of the basin
began in the Upper Carboniferous and progressed from the present west to east (Feng
et al., 2001). This closure was associated with the formation of an accretionary wedge
on the west side, in which the volcanogenic-hosted massive sulfide deposits were
sheared. The stockworks of the Dapingzhang and Sandashan deposits are strongly
tectonized and lie within the zone of the weakly- or unmetamorphosed accretionary
wedge. From this belt to the present west, the deformation increases parallel to the
gradual change from greenschist- (Phyllite Belt) to lower blueschist facies (Lancang
Group) documenting a large-scale thrust belt or accretionary wedge (in terms of
Sengör and Okurogullari, 1991).
[3] The closure of the basin led to the exhumation of the blueschist- and greenschist-
metamorphosed rocks and the formation of a land area on the western edge of the
IYangtze Platform up to the Middle Permian. The mineral resource potential related to
this stage is estimated to be low.
[4] Late Paleozoic orogenesis was followed by marine ingressions, post-orogenic
bimodal rift volcanism and continental sedimentation. The volcanism is an expression
of a regional thermal event causing crustal anatexis and the formation of the Lincang
Granite. The petrology and geochemistry of the basalts along the Lancang River are
identical to the continental flood basalts of the Emeishan Large Igneous Province (LIP),
which are proposed to be mantle-plume related (Xu et al., 2001; Chung, 1998). Hence,
the new plate tectonic model excludes a Mesozoic island-arc setting and expectations
of finding large-scale Au-bearing porphyry copper deposits cannot be met (Heppe and
Helmcke, 2003). Nevertheless, a close correlation between the volcanic rocks along
the Lancang River and the Emeishan flood basalts (and rhyolites) opens up a new per-
spective: Ore deposits in relation to large scale continental rifting, such as 1] Native Cu-
deposits of Keweenaw-type (Keweenaw rift, Precambrian, Superior Province,
Canada), 2] Ni-Cu-PGE deposits of Norilsk-Talnakh-type, Siberian Traps, or 3] Volcanic
red-bed Cu-deposits. The mineral deposit potential of the Lincang Granite is low.
Although the Lincang Granite has the chemistry of prospective tin granites, the similar-
ity to the North Thai Granite Province discourages a high mineral potential.
[5] In the Upper Triassic, weak compression and basin inversion took place along the
Lancang River Zone. The cause of this compression is unclear but could be associat-
ed with the collision of the Tengchong micro-continent further to the west. No mineral-
izations are known in relation to this event along the Lancang River Zone.
[6] A regional extensional phase took place from the Upper Jurassic to the Paleogene,
and the exhumation of the Lincang Metamorphic Core Complex controlled the deposi-
tion of massive sequences of continental red-beds in the Simao Region. The geome-
try of the basin can best be explained by a retreating plate boundary (in the sense of
Royden, 1993) to the west of the Tenasserim-Mogok Belt causing extension and the
formation of a retro-arc basin in the hinterland. Extension was accommodated along
predisposed anisotropies of the Late Paleozoic accretionary wedge and led to the for-
mation of west-dipping low-angle normal faults, along which cool lithospheric mantle
material was progressively replaced by hot asthenospheric mantle causing or enhanc-
ing a plateau-like uplift to the east of the Doi-Inthanon-Lincang Unit and a depression
in the Yunxian-Lanpang Unit (Min et al., 2001). In contrast to Yano et al. (1994), who
relates the source of the continental red-beds to the emplacements of granites in the
Mogok-Tenasserim Belt, this study regards the depositional features to be primarily
controlled by the exhumation of the Lincang Metamorphic Core Complex and the uplift
of the Doi-Inthanon Lincang Unit. In relation to this stage sediment-hosted stratiform
Cu- or red-bed Cu-deposits evolved.
[7] If the mechanics forming the Simao Basin are fundamentally correct, the local con-
sequence for western Yunnan is that a middle Cenozoic inversion of the Simao Basin
must have been largely accommodated by the readjustment of formerly extensional
allochthons, which progressively led to a complex interplay of thrusting, folding, fault-
ing and rotation. During this period a whole spectrum of mineralization developed as a
IIresult of the high fluid flow in the transtensional settings of the India-Asia collision in
West Yunnan. Copper porphyries occur in addition to polymetallic vein mineralization
and Au-quartz veins.
The results of this study refine and corroborate the regional geodynamic model as pos-
tulated by Helmcke (1985, 1986), Helmcke et al. (1993), and Ingavat-Helmcke and
Helmcke (1994). A differentiation between an “eo-” or “pre-Indosinian” (Late Paleozoic)
stage of mountain-building, that was related to high-pressure/low temperature metamor-
phism and convergence, and an Indosinian (Late Paleozoic) stage of low-pressure/high-
temperature metamorphism, that was related to extension and crustal anatexis, is
demonstratively recorded along the Lancang River Zone. On a regional scale, the cyclic
Cordilleran character of the evolution is further stressed by the Late Triassic accretion of
the Tengchong continental fragment, and the Jurassic to Early Tertiary exhumation of an
extensive N-S-striking belt of metamorphic core complexes with accompanying red-bed
basins (e.g. Simao Basin, Lanping Basin). The process that facilitated extension is spec-
ulated to be primarily controlled by a retreating active plate-boundary to the west of the
Jurassic to Miocene Mogok-Tenasserim Belt (continental island arc, Barley et al., 2003),
which parallels the core complexes and red-bed basins throughout Southwest Yunnan,
the eastern part of the Union of Myanmar (Burma), West Laos and northern Thailand. As
far as the Cretaceous development is concerned, the plateau-like uplift of the Doi-
Inthanon-Lincang Unit and the depression of the Yunxian-Lampang Unit may be further
traced to circum-Himalayan regions.
IIIContents
1. Introduction 1
1.1. Study Objectives 1
1.2. Preliminary Work by the Research Group in Yunnan 1
1.3. Location of the Study Are