Ewolucja składu mineralnego skał cieszyńskiej prowincji magmowej

Abstract

Igneous bodies that occur in the area of Cieszyn Silesiaand Morawy are characterised by an evolution that is typicalfor alkaline igneous complexes. The rocks display a widerange of composition; the majority are undersaturated withrespect to SiO2, while some show silica saturation, theamount of normative quartz not exceeding 10 weight percent.The K2O/Na2O ratio spans a wide range. Most of theCieszyn rocks show a sodic tendency, mineralogically expressedas a high abundance of normative nepheline.Potassic varieties are predominantly seen within melanocraticvariants that contain high proportions of phlogopiteand kaersutite.For the classification of the Cieszyn igneous province,the author proposes using the terms: picrite, picroteschenite,teschenite and syenite. Among mesocratic teschenitestwo varieties can be distinguished: an alkaline variety withnormative nepheline and a subalkaline variety as an equivalentof crinanite with normative quartz. For melanocraticteschenites the term clinopyroxenite teschenite or maficteschenite is proposed. A diabase structure is the characteristicfeature that distinguishes the subalkaline variety ofteschenite. The melanocratic (picroteschenites) and ultramafic(picrites) members of the Cieszyn rock series thatcontain olivine belong to an alkaline variety that displaysthe presence of the normative nepheline and leucite.Analysis of crystallochemical substitution in the mainmafic phases (pyroxenes, amphiboles, K-micas) by usingThompson’s exchange vectors unambiguously revealed theirsubsilica character and the primary role of Tschermak substitutions.On the basis of the analytical data gathered wecould conclude that both the temperature and the compositionof melts were the main factors driving the evolution oftheir composition in individual sills.Ca-rich pyroxenes that occur in all the rock types containhigh levels of Tschermak’s molecules. Their total maximumabundance in alkaline teschenites, picroteschenitesand syenites is within 32—36 %mol., decreasing in picrites(28 %mol.) and showing the lowest value in subalkalineteschenites (23 %mol). In all types of sill the evolution ofpyroxene composition has occurred towards the aegirinecomposition, with different ratios of Fe2+/Mg and in twostages of Ti enrichment.The composition of amphiboles varies from kaersutite(pargasite/magnesiohastingite) through ferrokaersutite tohastingite and towards NaCa-amphiboles (taramite,katophorite, richterite, winchite). In general, the trend inthe compositional variation of amphiboles approaches theNa-amphibole field (arfvedsonites). During the compositionalevolution towards Na-enrichment there was a decrease inthe amount of Ti and Al and an increase of Fe3+ and Si. Therocks analysed contain K-mica that displays a compositionalevolution as follows: phlogopite ⇒ biotite ⇒ annite; in nephelinesyenite veins an additional annite-syderophyllite trendwas identified.Two general trends in the evolution of pyroxenes andamphiboles of the Cieszyn igneous province were distinguished.Trend 1 depicts the evolution through the hedenbergite(ferrokaersutite) composition field with low valuesof fO2; whereas trend 2 omits this field and involves athigher fO2. The first trend was documented in sills of theraliteand subalkaline teschenites, the second one was reportedfrom essexite teschenites, picroteschenites andpicrites. The second trend in comparison with the first onereveals a constant Di-Ae evolution in melts enriched involatiles. Both trends distinguished were supported by theanalysis of the sodic pyroxenes examined. A Fe2+-NAT enrichmentwas observed for the first one, and a Mg-NAT enrichmentfor the second one. Additionally, the analysisrevealed the multi-pressure genesis of mafic phases as wellas a substantial influence of sector zoning on the fractionalcrystallisation within individual sills. Their compositions indicatethat the source magmas were melted frommetasomatised upper mantle (of the lithosphere). This processwas documented in the xenoliths of the Miedzyrzeczepicrite sill, where replacement of mantle Cr-diopside byMg-richterite was observed.Individual sills were subjected to the intensive fractionalcrystallisation of Ca- and Al-rich phases (cpx, amf, pl); resultingin the presence of syenite nepheline melts (SiO2-undersaturated)or quartz syenite melts (SiO2-saturated) as endproducts. With the decrease of MgO in the residual meltsthere was a simultaneous increase of Si, Al, Na and K. Thepresence of Na-pyroxenes (aegirine) and amphiboles(arfvedsonite) is an indicator of the per-alkaline compositionof melts. In addition, melts have reached the stage of zirconsaturation (the presence of a stand-alone zircon phase).Analogues to other alkaline igneous provinces; with theincrease of peralkalinity of residual melts, conditions of theT-fO2 for the non-oxide field were attained; in the syenitenepheline veins, Fe and Ti were incorporated into the compositionof titanites, NaCa-pyroxenes, amphiboles and annitebut with no aenigmatite contribution.During the crystallisation of the Cieszyn igneous provincethere were two regimes of oxygen activity in the melt.In the first regime the fO2 oscillated around the QFM buffer;whereas in the second regime this parameter could raise itsvalue to the level in between NNO and HM buffers. A verywide temperature range for in situ crystallisation in individualsills was documented. The main consolidation stage ofthe sill took place within the 1260—950(900)°C temperaturerange and P ≤ 2 kbar. In turn, the evolution of the interstitialmelts of alkaline syenite composition took place attemperatures of 950(900)—700°C.Signs of amphibolitisation of the mantle Cr-diopside indicatea metasomatic genesis of Mg-richterite and that theprocess was accomplished in the upper mantle rather thanin the asthenosphere. The process of amphibolitisation isa result of the infiltration of the upper mantle material byfluids and/or by volatile-rich melts that were generated bythe uprising lower mantle plume. This process decreasedthe temperature of the solidus of the metasomatised mantle;the temperature was attained by the thermal conductivity ofthe uprising mantle plume. The process of interaction betweenthe mantle plume and sub-continental upper mantletook place in the early stage of the Silesian Basin development,during rifting of the passive margin of the EuropeanCraton. The model of the development of magmatism in theSilesia Basin assumes an interaction between SCLM (shallow,sub-continental mantle) and uprising sub-lithosphericdiapirs. The PM process of metasomatised SCLM wouldhave had a two-stage character, that would be followed bya multi-pressure FC process [I FC for P < 7(8) kbar, < 30km; II FC in individual sills for P < 1(2) kbar, < 6 km].Melts that differentiation led to the formation ofSiO2-undersaturated rocks (alkaline teschenites) could havebeen melted in the first stage [PM < 3(4) %]. Melts thatare saturated with respect to silica have generated duringhigher degree of SCLM melting [PM < 5(6) %], this processhas probably also involved non-metasomatised fragmentsof the upper mantle. The generation of magma wouldhave occurred at the depths of 70—80 km.