SILICEOUS XENOLITHS IN THE SAND HOLLOW BASALT, FRANKLIN COUNTY, WA

Small, rounded to irregularly shaped siliceous xenoliths occur within the Sand Hollow basalt, Frenchman Springs Member, near Harsha, WA. Two populations of xenoliths are present in the flow: one population consists of microcrystalline to fine-grained quartz; the other population consists of glass containing subrounded fine grains of quartz with minor plagioclase and K-feldspar. SiO2 is dominant in the crystalline xenoliths; SiO2 along with significant Al2O3, Na2O and K2O are present in the glassy xenoliths. Undulatory extinction is present in the larger quartz grains from both xenolith types. Notably absent in both types of xenoliths are “triple junction” type grain boundaries that would suggest equilibrium following thermal recrystallization. Lava is present within some fractures in the crystalline xenoliths; minor glass in these xenoliths occurs along some fractures and along the xenolith margins. Lava has extensively penetrated the glassy xenoliths.

Contacts between the xenoliths and the basalt vary from sharp to diffuse suggesting a range in lava-xenolith interactions. Plagioclase laths in the basalt are commonly aligned parallel to the contact with the xenolith. Possible chilling of the basalt against the crystalline xenoliths is suggested by an increase in glass content within the basalt as the contact is approached.

The chemical composition of the glassy xenoliths suggests that they may represent melts derived from an argillaceous protolith. In contrast, the quartz-rich xenoliths appear to represent sandstones and cherts. The rounded character of many of the xenoliths suggests that they were present as stream gravel that was incorporated into the lava as the Sand Hollow flow advanced through a stream channel. The more extensive melting of the “argillaceous” xenoliths suggests that their protoliths had a lower melting temperature.