Zircon ages of granites occurring along the central swedish gravity low

Abstract

Two major types of late Svecofennian granitoids occur in south central Sweden. Large homogeneous massifs of coarse‐grained granite with trace element characteristics typical of granites formed in tensional settings occur along the Central Swedish Gravity Low (CSGL). The other type comprises smaller intrusions of irregular, heterogeneous, locally derived granites (IHLD granites). We present U‐Pb zircon ages of one IHLD granite and of three varieties of the homogeneous intrusions. The result for the IHLD granite is 1779±8 Ma. This age together with previously published results indicate that various types of leucocratic irregular masses of anatectic IHLD granites and associated pegmatites were formed discontinuously over a period so far bracketed by the ages 1846+1 Ma and 1779±8 Ma, suggesting the existence of thickened crust for a period of at least 70 Ma. It is probable, however, that only the older of these granites were associated with regional migmatitization. One of the samples of the homogeneous type yielded an age of 1769.7±3.4 Ma. The remaining two samples have heterogeneous zircon populations. The emplacement age of these rocks is bracketed by the intercept ages of 1770±6 Ma and 1779±23 Ma, and the apparent ²⁰⁷Pb/^2O6Pb age of 1786 Ma from a zircon fraction that contains inherited components. Granite formation culminated with the genesis of the large, homogeneous intrusions of deep‐seated granite, which represent the largest volume of late Svecofennian granitic rocks in south central Sweden. These granites seem to have been formed during a narrower time‐interval, between c. 1800 and 1770 Ma. Granite formation along the CSGL ceased after this culmination. The overlapping ages of granite groups of different types show that different types of granites were formed contemporaneously in the same region as a results of melt formation at various depths and from various sources.

Öhlander, B. & Romer, R.L., 1996: Zircon ages of granites occurring along the Central Swedish Gravity Low. GFF, Vol. 118 (Pt. 4, December), pp. 217–225. Stockholm. ISSN 1103–5897.

Keywords:

• Zircon age
• U‐Pb age
• late Svecofennian
• Central Swedish Gravity Low
• homogeneous granite
• IHLD granite