Chemical Mineralogy Of The Yaojiazhuang Alkaline Complex And Constraints On The Formation Of Spheroidal Texture

Globally,many magmatic lithologies including both extrusive and intrusive rocks do carry spheroidal textures,however,their origins remain poorly understood and hotly debated,so it is of great petrological significance to investigate this unique texture.Yaojiazhuang complex is concentrically zoned by late-intruded syenite in the core and early emplaced clinopyroxenite in the periphery,combining a“bi-modal”lithology.The spheroidal texture rocks surround the syenite and set between the two lithologies with globules compositionally like syenite and matrix like clinopyroxenite.Our investigation found that the globules contain abundant micro-existing K-feldspar,nepheline,muscovite,analcite and celestine,and the matrix includes mainly clinopyroxene,biotite garnet and magnetite,exhibiting different mineral assemblages.The distribution of crystals is heterogeneous in the globule,with anhedral zoned K-feldspar presented in lineation.The clinopyroxenes in the matrix generally show complex zonations,and orient with biotite along the boundary of globules.The globules also exhibit orientation and gradually aggregate from inner side of the complex to the outer side.Geochemically,the compositions of Yaojiazhuang samples are not plotted within the two-liquid field,and compared to the syenite,the globule are poorer in SiO₂ but concentrate more Al₂O₃ and LILE.Many evidences in favour of a water exsolution model but not silicate liquid immiscibility in previous studies.Firstly,clinopyroxene is the first mineral to reach its liquidus in an early emplaced magma chamber which is relatively enriched in water and potassium,resulting in a rigid crystal mush with its interstitial melt compositionally equals to biotite+K-feldspar.When a fluid-rich trachytic?syenitic?melt intruded into this magma chamber,due to the relief on pressure,abundant water would be exsolved?melt-fluid immiscibility?and form water bubbles within the syenitic magma chamber.These bubbles would selectively carry those crystallised crystals like alkali feldspars and be released into the wall rock—the clinopyroxenite crystal mush.Calculated results show that the silicate constituent in the exsolved mixture is equal to a phonolite with about 10 wt.%H₂O,less CO₂?⁰.5 wt.%?and some sulphur.Following thermodynamics,rejuvenation?remelting?would thus happen due to the addition of fluids which can lower the solidus of clinopyroxene and biotite in the crystal mush,and the entirety would reach a condition more like“melts”.When the K-feldspar in the exsolved mixture encountered the“cold”crystal mush,the micro co-existing texture of K-feldspar,nepheline and muscovite.Along with the crystallization proceeds,the over-saturated fluids in globules exsolved again and carried hydrophilic minerals?clinopyroxene&biotite?into the matrix,leaving K-feldspar in the globule to form the spheroidal texture rocks,during which also cause the difference between globule-matrix and syenite-clinopyroxenite on geochemistry.Aqueous fluid also migrated upward by buoyancy within the syenitic magma chamber as degassing-induced crystallization proceeds,carrying abundant soluble elements?Na&Ca?into the top of the chamber.The relatively hydrous zone on top formed Na-rich syenite,less aegirine syenite and syenititc pegmatite.The melts below the hydrous zone and solidified as syenite,compositionally similar to orthoclase due to the lost on Na-Ca.Our investigation shows the exsolution of water by reactions between the syenitic magma and the clinopyroxene crystal mush under an open system,the fluids migration within the crystal mush play very important roles in the formation of Yaojiazhuang ultrapotassic intrusion.Moreover,the migration of aqueous fluids is of great possibility to be the key factor for those syenites obstructed to evolve to nepheline syenites.