Origin Of Syenite Research Articles

Mineralogical and geochemical constraints on origin of Paleoproterozoic clinopyroxene syenite in Trans-North China orogen

Syenites are commonly associated with alkaline silicate rocks and carbonatites. The origin of syenites, however, remains controversial. Here, we report an intrusive suite of carbonatite–clinopyroxenite–clinopyroxene syenite complexes and granites from the Trans-North China orogen in the North China craton and present the systematic mineralogy and geochemistry of the syenites and their associated clinopyroxenites and granites. The granite has an age of 1808 ± 6 Ma and εHf(t) ranging from −5 to −8.5, which is consistent with the clinopyroxene syenite. The syenite is composed mainly of Ba–Sr-rich alkali-feldspar and clinopyroxene, and is strongly associated with the clinopyroxenite and granite. The rock exhibits a plastic flow feature in the contact zone between the clinopyroxenite and granite. In the contact zone towards the granite margin, the alkali-feldspar shows gradual decreases in Ba, Sr, and rare earth elements (REE), whereas the clinopyroxene shows good transition from diopside to hedenbergite. The syenitic clinopyroxene has variable Fe–Mg and REE contents. Differences in the compositional zoning of the alkali-feldspar and clinopyroxene were observed such that the former mineral core contains relatively lower Sr (1864 vs. 11754 ppm), Ba (8590 vs. 41504 ppm), and REE (1.0 vs. 23 ppm, ∑La–Eu) abundances than the rim, whereas the clinopyroxene shows the opposite trend [1086 vs. 847 ppm for Sr, 178 vs. 85 ppm for REE (∑La–Lu) in the core and rim, respectively]. The mineral chemistry implies the clinopyroxene syenite was formed by mixing between the mantle-derived clinopyroxenite and crust-derived granite melts. The age of rock formation is close to the metamorphism age of high-pressure granulites in the Trans-North China orogen, indicating a post-collisional setting related to the inter-continental collision of the North China craton in the Paleoproterozoic.

The system fayalite-albite-anorthite and the syenite problem

The presence in a magma of fayalite, the iron end-member of the olivine binary series, affects the feldspars at pressure by lowering the temperatures at which they crystallize from the magma. Starting with estimates from published literature it becomes obvious that at pressure, fayalite becomes important because the pressure effects on the melting temperatures are very different: large for albite, and small for anorthite. In this experimental study, a powder of fayalite composition was combined with six finely ground natural feldspars from Ab to An 97 to make six bulk compositions. Using graphite crucibles in piston-cylinder apparatus at a pressure of 5 kbar, a cotectic in the ternary system was found to range from 1141 °C at An(Fa) to 1124 °C at Ab(Fa), with fayalite contents from 68 to 17 wt%, respectively. The results can be used to show that ternary feldspars saturated with fayalite and Fe monoclinic pyroxene will crystallize at a 5 kbar multiphase eutectic 1010 °C—56 °C below a calculated azeotropic point on the Ab-Or join. The results are used to compare the end points of two very different layered intrusions, Skaergaard and Kiglapait, and to illuminate the nature and origins of syenite and trachyte, which are leucocratic rocks unsaturated with mafic minerals. Because fayalite-saturated melts are responsive to pressure unequally on the feldspar end-members, olivine of intermediate composition will have a damped but potentially significant effect on feldspar fractionation in the lower crust of the Earth, possibly affecting the origin of anorthosite and syenite.

Potassic Feldspar Enrichment in Magma: Origin of Syenite in Deboullie District, Northern Maine

Research Article| December 01, 1962 Potassic Feldspar Enrichment in Magma: Origin of Syenite in Deboullie District, Northern Maine GARY M BOONE GARY M BOONE Dept. Geology, The University of Western Ontario, London, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information GARY M BOONE Dept. Geology, The University of Western Ontario, London, Canada Publisher: Geological Society of America Received: 31 May 1961 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1962, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1962) 73 (12): 1451–1476. https://doi.org/10.1130/0016-7606(1962)73[1451:PFEIMO]2.0.CO;2 Article history Received: 31 May 1961 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GARY M BOONE; Potassic Feldspar Enrichment in Magma: Origin of Syenite in Deboullie District, Northern Maine. GSA Bulletin 1962;; 73 (12): 1451–1476. doi: https://doi.org/10.1130/0016-7606(1962)73[1451:PFEIMO]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A small composite pluton, the Deboullie stock, comprises a post-orogenic, alkali-calc rock series intrusive into lower Devonian slate. The stock is a partly concordant, partly discordant pluton, and the wall rock structure shows that the magma was forcibly injected. The main intrusives are, from oldest to youngest, pyroxene-biotite syenodiorite and monzonite, pyroxene-hornblende-biotite calc-alkali syenite, and hornblende-biotite granodiorite. Dikes are the fine-grained equivalents of the salic plutonic rocks. Differentiation by fractional crystallization and by upward transfer of H2O and components of potassic feldspar in the melt is proposed as the only satisfactory hypothesis to account for the contrasts in mineralogic evolution. Accordingly, syenitic rocks were derived from the parent magma enriched in potassic feldspar at the top of the magma column; the syenitic magma graded into granodioritic differentiate at deeper levels. The granodioritic magma, richer in plagioclase and quartz, completed the emplacement of the stock by intruding the earlier emplaced calc-alkali syenite.Compositional differences across oscillatory zones in orthoclase and plagioclase feldspars of the main intrusive rocks apparently reflect changes in the partial pressure of volatiles in the melt. Intermittent fracturing of the roof of the magma chamber is appealed to as a cause for the pressure changes and as a mechanism for maintaining a steady-state gradient for H2O and K-bearing components.Neither assimilation, hydrothermal enrichment, nor liquid-crystal reactions involving the mafic minerals adequately explains the abundance of potassic feldspar in the syenite. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.