Geological Survey of Finland

Geological Survey of Finland Bulletin 303 Contribution to the mineralogy of rapakivi granites: I. Zircon of the Laitila rapakivi, southwestern Finland by Atso Vorma and Tuula Paasivirta Geologinen tutkimuslaitos Espoo 1979 Geological Survey of Finland, Bulletin 303 CONTRIBUTION TO THE MINERALOGY OF RAPAKIVI GRANITES: 1. ZIRCON OF THE LAITILA RAPAKIVI, SOUTHWESTERN FINLAND BY ATSO VOR MA AND TUULA PAASIVIRTA 30 figures and 5 tables in the text GEOLOGINEN TUTKIMUSLAITOS ESPOO 1979 Vorma, Atso & Paasivirta, Tuula, 1979: Contribution to the mineralogy of rapakivi g ranites: I. Zircon of the Laitila rapakivi, southwestern Finland. Geological SlIrvey 0/ Finland, Bulletin 303. 40 pages, 30 figures and 5 tables. The hafnium content and the Hf/Zr ratio in zircon indicate a progressive enrichment of hafnium in relation to zirconium with advancing differentiation of the Laitila rapakivi granite. The Hf/Zr ratio in zircon is about 0.023 in the prevailing rapakivi variety, which is the normal rapakivi granite and occupies more than 90 per cent of the massif. In the rock itself, it is 0.025, indicating possibly a very small fractionation of hafnium and zirconium be tween the z ircon and other minerals. The contact varieties contain zircon with a Jower Hf/Zr ratio than the sampIes farther from the contact. When zoning is detected in the zircon, the cores are usuall y poorer than the marginal parts in hafnium. Zirco ns in Ji ghter density fractions show a higher Hf/ Zr ratio than those in the heavier fractio ns. In addition, it is most Jikel y that the zircons in the lighter density fractions are richer in uranium and therefore susceptible to metamictizati on. The study is based on 331 microprobe anaJyses of hafnium and zi rconium in zircon and 37 instrumental activation analyses of these elements in the corresponding rock sampJes. Key words: rapakivi, granite, mineraJogy, geochcmistry, zircon, zirconium, hafnium, meta miet, magma, crystallization, differentiation, FinJand. Atso Vorma and Tllllia Paasivirta Geological SlIrvCJ' 0/ Finland, SF-02 150 E spoo 15, Fillland ISBN 951-690-109-3 ISSN 0367-522x Helsinki 1979. Valtion painatuskesklls CONTENTS Introduction .. .. .......................... ..... .................. .......... ... :; Laitila rapakivi . .. ............................ .................... ..... ......... 5 Rock sampie description . .... ....... ...... .......... .. ..... 7 Sampling . ....... .. ..... ....... ... ......... ... .. ............... ...... .... 7 Zirconium and hafnium contents . ......... .. .. .. .... ......... .... .... ... 10 Mode of occurrence of zircon in rapakivi . .. 11 Abundance of zircon in rapakivi ....... .. ... ... .............. ....... .. ... 14 Zircon analysis ..... .................. ..... .. ...... ............................ 15 Sampie preparation and description ...... .. ....... ... .... ... ......... ... 15 Methods ... .................. ... .... ... .. .. , . .. .. .. .. .. .. .. 20 Results .... .. ............. .. ...... ..... .. .. ........ .. ..... .. .. .. .. 21 Zircon composition vs. rock composition ....... .. ........ .. .................... 21 Hf/Zr ratio ..... .... ....... ........................... ... .... ....... .... 21 HfO 2 + Zr0 2 vs. Si0 2 •••••. • •• ••••••••• • • •••...••.•. •••. .. .••••••• • •• • •••• . • 31 Discussion ...................... .. ...... ....... ........ .... ... ............ 31 Acknowledgments ...... .... .. .. .. ... .... ....... .................. ... .. ...... 38 References ... .... .. ........ ......... .... ... ................. ............ 39 INTRODUCTION During recent yea rs, rapakivi granite has re 1977, gave a thorough description of the north ceived considerable attention in Finland not western part of the massif. Vaasjoki, also in merely because of its attractive texture (see, 1977, discussed the age relations between dif e.g., Vorma 1971) but also owing to its mineral ferent rapakivi varieties of the Laitila massif. ogy and petrology (Sirnonen and Vorma 1969, These studies form the background to the Vorma 1971, 1972, 1975 and 1976, and Haapala present study. 1977 a), metallogeny (Haapala 1973, 1974, The purpose of this study was to determine 1977 a and 1977 b, Haapala and Ojanperä 1969 whether any correlation between the Hf/ Zr ratio and 1972), and geochronology (Vaasjoki 1977). in zircon and the differentiation of the massif The Laitila rapakivi massif in southwestern could be found. With this in view, 331 micro Finland is at present the main object of the probe analyses of hafnium and zirconium from rapakivi studies of one of the present authors zircons and 37 instrumental neutron activation (A. V.). An important part of these studies is analyses of hafnium and zirconium from the the investigation of accessory minerals. Among corresponding rock sampies were performed. In the accessory minerals characteristic of rapakivi, addition, silicate analyses made from the rock zircon is of utmost importance. The study sampies are available. therefore begins with zircon. Similar studies on Atso Vorma is responsible for providing the apatite, monazite, allanite, etc., will be under sampIes and for setting up the problems posed taken soon. by this study, and Tuula Paasivirta for the The general evolution of the Laitila massif microprobe analyses. The manuscript was com was traced by Vorma in 1976. Haapala, in piled and the condusions were drawn jointly. LAITILA RAPAKIVI The Laitila massif, which comprises an area but doser in mineral and chemical composition of ca. 1 400 sq. km in southwestern Finland, to wiborgite than pyterlite. The ovoidal text ure is a composite pluton. The rock types and their is weil developed, and only thin plagiodase mutual relations can be briefly characterized, mantles around alkali feldspar ovoids are sparsely after Vorma (1976, pp. 37- 56): present. The normal Laitila rapakivi is either The bulk of the massif consists of normal a hornblende-biotite rapakivi or biotite rapa Laitila rapakivi, which is similar in texture to kivi. It is highly probable that these different the dassical pyterlite of the Wiborg massif varieties represent different intrusive phases. 6 Geological Survey of Finland, Bulletin 303 17 features so much that they have been designated o 16 r by special names. Some of these are evidently I oE an autolithic nature formed Erom magma 15 chilled earlier and brecciated by the same AI~03 14 magma. Apart certainly is a result of early segregation. Some oE the granites were evidently 13 produced by autointrusion. Finally, some oE 12 the small granite bodies, as, Eor instance, those 11 oE Ytö and Suutila (see Vorma 1976, Plate 1), formed Erom intrusion phases different from FeO tot \:-;-- those represented by the normal rapakivi. 6 The Leliainen-type, even-grained biotite rapa kivis form a quite coherent group in the eastern part of the massif. In places, they seem to change gradually to the normal Laitila rapa kivi, but in other places the contact is sharp. 3 A contact variety occurs in certain places against 2 the normal rapakivi. All these features can be understood if these granites are taken to be a product oE differentation oE the normal rapakivi o ~--------------------------------~ (magma) and oE autointrusion. 3 So-calied spotted granites form a special 2 group among the biotite rapakivis. It is charac teristic oE these granites that there are abundant black biotite aggregates in the small or medium grained matrix. The occurrences oE spotted rapakivi are small. Being possibly of anatectic origin, the rock is evidently younger than the normal rapakivi. ~?e~~ßtZ- Distinct Erom the Eoregoing biotite rapakivis, 65 70 75 80 there are certain coarse- to fine-grained biotite 1 1 ••• ___ .... Torkki gr Normal rOpOklYI Väkkäro gr . granites . t and hornblende-biotite granites that occur as I Var ious biotite ~tÖ.,, ; . ropokivis small bodies in the normal Laitila rapakivi . Some oE these are evidently produced by auto Fig. 1. Harker diagram of the granitic rocks intrusion from the same intrusive phase Erom from the Laitila rapakivi massif (from Vorma 1976, p. 58). which the normal Laitila rapakivi crystaliized. Some are derived Erom thoroughly different intrusive phases, such as, e.g., the Ytö granite and related rocks. Their emplacement evidently Different textural varieties also occur, such as, took place by the mechanism oE underground e.g., even-grained marginal granite and granite cauldron subsidence. According to Vaasjoki porphyritic, as weil as evidently marginal granite. (1977), the age of these granites is 1 540 ± 10 Ma A minor part of the Laitila massif consists while the age oE normal Laitila rapaki vi is of different rapakivi varieties. These deviate 1 573 ± 8 Ma. The Ytö granite and related from the normal Laitila rapakivi in their textural rocks constitute two concentric bodies, in which Geological Survey of Finland, Bulletin 303 7 the marginal granites are coarse-grained and and a younger central biotite granite, called porphyritic, the central granites small-grained Väkkärä granite. A detailed description of this and aplitic. In the Ytö complex both the mar complex can be found in Haapala 1977 a. ginal and the central granite are called Ytö The Laitila massif and its country rocks are granites. In the Katinhäntä-Suutila complex the cut by different kinds of rapakivitic dike rocks: marginal granite is known as Suutila granite pegmatite-, granite-, aplite-, porphyry aplite-, and the central granite Katinhäntä aplite. A granite porphyry-, quartz porphyry- and quartz feature peculiar

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