Clinopyroxene from basaltic rocks of the Erzgebirge - Kru né hory Mts. - implications for modelling the magmatic plumbing syste.docx

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1、Journal of Geosciences, 55 (2010), 231 - 249DOI: 10.3190/jgeoscL077Original paperClinopyroxene fro m basaltic rocks of the Erzgebirge-Krusne horyM ts. - implications for modelling of the mag matic plu mbing systemRoman RONICK , Axel D. RENNO1 1*Technische Universitdt Bergakademie Freihergf Institute

2、 of Mineralogy, Brennhausgasse 149 D-09596 Freiherg9 Germany;axel. rennoniineral. tu-freiherg. de* Corresponding authorThe Erzgebirge-Krusne hory area is part of the NW flank of the Eger-Ohre Rift. Within this rift-related area, basalticrocks with low ( 3.5 wt. %) of TiO? occur. The distribution of

3、these basaltic rock typ-es shows strong spatial dependency. Ti-rich basalts are concentrated in the western Erzgebirge-Kmsne hory Mts. with asharp change at longitude 13.3 0E. Clinopyroxene and olivine are the most abundant phenocrysts. The zoning and che-mistry of the clinopyroxene act as a recorde

4、r of the dierent stages of the magmatic plumbing systems. Seven differenttypes of clinopyroxene phenocrysts are defined with four distinct chemical types: Cr-rich clinopyroxene, green-core py-roxene, Mg-rich clinopyroxene and finely zoned Ti-rich clinopyroxene with sector zoning.Chemical and petrogr

5、aphic analysis of individual zones within clinopyroxenes allows the qualitative description of dis-tinct steps of the multistage, regionally diverse processes yielding to the formation of both Ti-rich basalts in the W aswell as Ti-poor basalts in the E. Two different types of asthenospheric melts ar

6、e suggested, with increased activities ofH2O and potassium in the western Erzgebirge Mts. During ascent they incorporated xenoliths and xenocrysts ofboth themantle (peridotite, olivine and Cr-rich clinopyroxene) and the crustal (granitic and gncissic rocks, quartz and feldspar)origin. Furthermore, g

7、reen cores of clinopyroxene were introduced into the magmas. Fractionation processes took placewithin lithospheric magma chambers, having been accompanied by reaction of xenocrysts and xenoliths with the melt.The residence time in the magma chambers associated with the eastern and western parts of t

8、he Erzgebirge appears tohave been totally different: short in the eastern and long in the western parts. Within the latter, no mantle-derived xe-noliths, xenocrysts, or magmatic olivines are found. It is inferred that these xenoliths and crystals settled to the bottomof the magma chambers forming cu

9、mulates and therefore did not come to the surface. In contrast, the basaltic rocks ofthe East still contain mantle-derived xenoliths and xenocrysts as well as magmatic olivine. The size and amount of cli-nopyroxene phenocrysts in the West significantly exceed that of the East. Moreover, the crystall

10、ization of giant clino-pyroxenes was restricted to the western region. Mixing of the magmas with Ti-rich melts, presumably derived from aneclogitic source, took place in crustal magma chambers. In any case, the proportion of Ti-rich melts in the West signi-ficantly exceeded that of the East.Keywords

11、: petrology, crystal chemistry, clinopyroxene, basalt, EfzgehirgeKrusne hory, green-core pyroxeneReceived: 6 April 2010; accepted: 20 September 2010; handling editor: V. Rapprich/R. SkalaThe online version of this article (dot: 10.3190/jgeosci. 077) contains supplementary electronic material.1. Intr

12、oductionDespite the eye-catching geochemical character ofTi-rich basalts it is unclear if this feature is a vagary ofnature or an important geochemical fingerprint able todiscriminate between different source regions of basal-tic rocks. Besides well known High-Ti basalts from themoon (Papike et al.

13、1976; Papikc and Vaniman 1978),which have no terrestrial equivalent, High-Ti basalts arcfound in different gcotcctonic settings, such as Continen-tal Flood Basalts (CFB), Large Igneous Provinces (LIP)(Gibson ct al. 1995), or rift-likc intraplatc settings (Pik ctal. 1998). The occurrence of such basa

14、lts is not restrictedto lavas; they appear also in dike systems (Essawy andEl-Metwally 1999; Katzir ct al. 2006) and intrusions ofgabbroic rocks (Jama Aden and Frizzo 1996). Their agespans from the Archaean to recent. The origin in each ofthe geotectonic environments remains controversial. Thepropos

15、ed explanations in LIPs range from a high degreeof fractionation (Fodor 1987), low degree of partial melt-ing (Dupuy et al. 1988), crustal contamination (Fodor1987), to the influence of subcontinental lithospheremobilized by subduction (Duncan 1987; Murphy 1988).It is assumed that the effect of mant

16、le plumes is the maincause for the genesis of Ti-rich rocks in rift-like settings(Pik et al. 2006).A qualitative approach to understanding petrologicalprocesses responsible fbr the genesis of Ti-rich basaltsthrough the comprehensive study of the microstructure andmineral chemistry of clinopyroxene is demonstrated in the